View source for Actinide

{{short description|F-block chemical elements}}
{{Use dmy dates|date=May 2020}}
{{Periodic table (micro)| mark=Ac,Th,Pa,U,Np,Pu,Am,Cm,Bk,Cf,Es,Fm,Md,No|title=Actinides in the [[periodic table]]}}
{{Sidebar periodic table|expanded=metalicity}}

The '''actinide''' ({{IPAc-en|ˈ|æ|k|t|ᵻ|n|aɪ|d|}}) or '''actinoid''' ({{IPAc-en|ˈ|æ|k|t|ᵻ|n|ɔɪ|d|}}) series encompasses at least the 14 metallic [[chemical element]]s in the [[Electron shell|5f series]], with [[atomic number]]s from 89 to 102, [[actinium]] through [[nobelium]]. Number 103, [[lawrencium]], is also generally included despite being part of the 6d transition series. The actinide series derives its name from the first element in the series, actinium. The informal chemical symbol '''An''' is used in general discussions of actinide chemistry to refer to any actinide.<ref name="Gray">{{cite book|author=Theodore Gray|title=The Elements: A Visual Exploration of Every Known Atom in the Universe|year=2009|publisher=Black Dog & Leventhal Publishers|location=New York|isbn=978-1-57912-814-2|page= 240|url=https://archive.org/details/elementsvisualex0000gray/page/240}}</ref><ref>{{cite web|first1=Lester|last1=Morss|first2=Larned B.|last2=Asprey|url=https://www.britannica.com/science/actinoid-element |title=Actinoid element |publisher=Encyclopædia Britannica|date=1 August 2018|website=britannica.com|access-date=3 September 2020}}</ref><ref>{{cite book|author=Neil G. Connelly|title=Nomenclature of Inorganic Chemistry|publisher=[[Royal Society of Chemistry]]|location=London|year=2005|chapter-url=https://books.google.com/books?id=w1Kf1CakyZIC&pg=PA52|page=52|chapter=Elements|isbn=978-0-85404-438-2|display-authors=etal}}</ref>

The 1985 [[IUPAC nomenclature of inorganic chemistry|IUPAC ''Red Book'']] recommends that ''actinoid'' be used rather than ''actinide'', since the suffix ''-ide'' normally indicates a [[negative ion]]. However, owing to widespread current use, ''actinide'' is still allowed.

Actinium through nobelium are [[f-block]] elements, while lawrencium is a [[d-block]] element<ref>{{cite journal|author1-link=William B. Jensen|last1=Jensen|first1=William B.|date=2015|title=The positions of lanthanum (actinium) and lutetium (lawrencium) in the periodic table: an update|url=https://link.springer.com/article/10.1007/s10698-015-9216-1|journal=Foundations of Chemistry|volume=17|issue= |pages=23–31|doi=10.1007/s10698-015-9216-1|s2cid=98624395 |access-date=28 January 2021|url-access=subscription}}</ref><ref>{{cite journal|last1=Scerri|first1=Eric|date=18 January 2021|title=Provisional Report on Discussions on Group 3 of the Periodic Table|journal=Chemistry International|volume=43|issue=1|pages=31–34|doi=10.1515/ci-2021-0115 |s2cid=231694898 |doi-access=free}}</ref> and a [[transition metal]].<ref name=Neve>{{cite journal |last1=Neve |first1=Francesco |date=2022 |title=Chemistry of superheavy transition metals |url= |journal=Journal of Coordination Chemistry |volume=75 |issue=17–18 |pages=2287–2307 |doi=10.1080/00958972.2022.2084394 |s2cid=254097024 |access-date=|doi-access=free }}</ref> The series mostly corresponds to the filling of the 5f [[electron shell]], although as isolated atoms in the ground state many have anomalous configurations involving the filling of the 6d shell due to interelectronic repulsion. In comparison with the [[lanthanide]]s, also mostly f-block elements, the actinides show much more variable [[valence (chemistry)|valence]]. They all have very large [[atomic radius|atomic]] and [[ionic radius|ionic radii]] and exhibit an unusually large range of physical properties. While actinium and the late actinides (from [[curium]] onwards) behave similarly to the lanthanides, the elements [[thorium]], [[protactinium]], and [[uranium]] are much more similar to [[transition metal]]s in their chemistry, with [[neptunium]], [[plutonium]], and [[americium]] occupying an intermediate position.

All actinides are [[radioactive]] and release energy upon radioactive decay; naturally occurring uranium and thorium, and synthetically produced plutonium are the most abundant actinides on Earth. These have been used in [[nuclear reactor]]s, and uranium and plutonium are critical elements of [[nuclear weapon]]s. Uranium and thorium also have diverse current or historical uses, and americium is used in the [[ionization chamber]]s of most modern [[smoke detector]]s.

Due to their long half-lives, only thorium and uranium are [[Primordial nuclide|found on Earth]] and astrophysically in substantial quantities. The radioactive decay of uranium produces transient amounts of actinium and protactinium, and atoms of neptunium and plutonium are occasionally produced from [[Nuclear transmutation|transmutation]] reactions in [[uranium ore]]s. The other actinides are purely [[synthetic element]]s.<ref name="Gray" /><ref name=g1250>Greenwood, p. 1250</ref> Nuclear weapons tests have released at least six actinides heavier than plutonium into the [[natural environment|environment]]; analysis of debris from the [[Ivy Mike|1952 first test]] of a [[hydrogen bomb]] showed the presence of americium, [[curium]], [[berkelium]], [[californium]], and the discovery of [[einsteinium]] and [[fermium]].<ref name="PR1955">{{cite journal|last1=Fields|first1=P.|last2=Studier|first2=M.|last3=Diamond|first3=H.|last4=Mech|first4=J.|last5=Inghram|first5=M.|last6=Pyle|first6=G.|last7=Stevens|first7=C.|last8=Fried|first8=S.|last9=Manning|first9=W.|last10=N.N.|title=Transplutonium Elements in Thermonuclear Test Debris|journal=Physical Review|volume=102|issue=1|pages=180–182|year=1956|doi=10.1103/PhysRev.102.180|bibcode=1956PhRv..102..180F|display-authors=9}}</ref>

In presentations of the [[periodic table]], the f-block elements are customarily shown as two additional rows below the main body of the table.<ref name="Gray" /> This convention is entirely a matter of aesthetics and formatting practicality; a rarely used wide-formatted periodic table inserts the 4f and 5f series in their proper places, as parts of the table's sixth and seventh rows (periods).

<div style="background:{{element color|table background}}; border:1px solid {{element color|table border}}; margin:0; padding:2px; text-align:center; vertical-align:top; width: 66em; max-width: 100%">
<div style="background:{{element color|table title}}; padding:2px 4px;font-weight:bold; font-size: 90%">Actinides</div><!--
-->
{| cellpadding="0" cellspacing="1" style="overflow-x: auto; display:block; border-collapse: separate!important; table-layout:fixed; width: 100%; margin: 0!important;"
| width=6.6% style="min-width: 3em" | {{element cell-named|89|actinium    |Ac|[227]|solid|f-block|from decay}}
| width=6.6% style="min-width: 3em" | {{element cell-named|90|thorium     |Th|     |Solid|f-block|Primordial}}
| width=6.6% style="min-width: 3em" | {{element cell-named|91|protactinium|Pa|     |Solid|f-block|From decay}}
| width=6.6% style="min-width: 3em" | {{element cell-named|92|uranium     |U |     |Solid|f-block|Primordial}}
| width=6.6% style="min-width: 3em" | {{element cell-named|93|neptunium   |Np|[237]|Solid|f-block|From decay}}
| width=6.6% style="min-width: 3em" | {{element cell-named|94|plutonium   |Pu|[244]|Solid|f-block|From decay}}
| width=6.6% style="min-width: 3em" | {{element cell-named|95|americium   |Am|[243]|Solid|f-block|Synthetic}}
| width=6.6% style="min-width: 3em" | {{element cell-named|96|curium      |Cm|[247]|Solid|f-block|Synthetic}}
| width=6.6% style="min-width: 3em" | {{element cell-named|97|berkelium   |Bk|[247]|Solid|f-block|Synthetic}}
| width=6.6% style="min-width: 3em" | {{element cell-named|98|californium |Cf|[251]|Solid|f-block|Synthetic}}
| width=6.6% style="min-width: 3em" | {{element cell-named|99|einsteinium |Es|[252]|Solid|f-block|Synthetic}}
| width=6.6% style="min-width: 3em" | {{element cell-named|100|fermium    |Fm|[257]|Unknown phase|f-block|Synthetic}}
| width=6.6% style="min-width: 3em" | {{element cell-named|101|mendelevium|Md|[258]|Unknown phase|f-block|Synthetic}}
| width=6.6% style="min-width: 3em" | {{element cell-named|102|nobelium   |No|[259]|Unknown phase|f-block|Synthetic}}
| width=6.6% style="min-width: 3em" | {{element cell-named|103|lawrencium |Lr|[266]|Unknown phase|d-block|Synthetic}}
|}
{{periodic table legend|theme1=occurrence|border1=13}}
</div>

== Discovery, isolation and synthesis ==
{| class="wikitable"  style="float:right; margin-left:1em;"
|+Synthesis of transuranium elements<ref name=g1252>Greenwood, p. 1252</ref><ref group=notes>Nobelium and lawrencium were almost simultaneously discovered by Soviet and American scientists</ref>
! Element
!Year
!Method
|-
| [[Neptunium]]
| align=center| 1940
| Bombarding <sup>238</sup>U with [[neutron]]s
|-
| [[Plutonium]]
| align=center| 1941
| Bombarding <sup>238</sup>U with [[deuteron]]s
|-
| [[Americium]]
| align=center| 1944
| Bombarding <sup>239</sup>Pu with neutrons
|-
| [[Curium]]
| align=center| 1944
| Bombarding <sup>239</sup>Pu with [[Alpha particle|α-particles]]
|-
| [[Berkelium]]
| align=center| 1949
| Bombarding <sup>241</sup>Am with α-particles
|-
| [[Californium]]
| align=center| 1950
| Bombarding <sup>242</sup>Cm with α-particles
|-
| [[Einsteinium]]
| align=center| 1952
| As a product of [[nuclear explosion]]
|-
| [[Fermium]]
| align=center| 1952
| As a product of nuclear explosion
|-
| [[Mendelevium]]
| align=center| 1955
| Bombarding <sup>253</sup>Es with α-particles
|-
| [[Nobelium]]
| align=center| 1965
| Bombarding <sup>243</sup>Am with [[Nitrogen-15|<sup>15</sup>N]] <br />or <sup>238</sup>U with [[Neon-22|<sup>22</sup>Ne]]
|-
| [[Lawrencium]]
| align=center| 1961<br />–1971
| Bombarding <sup>252</sup>Cf with [[Boron-10|<sup>10</sup>B]] or [[Boron-11|<sup>11</sup>B]]<br />and of <sup>243</sup>Am with <sup>18</sup>O
|}

Like the [[lanthanide]]s, the actinides form a family of elements with similar properties. Within the actinides, there are two overlapping groups: [[transuranium element]]s, which follow uranium in the [[periodic table]], and transplutonium elements, which follow plutonium. Compared to the lanthanides, which (except for [[promethium]]) are found in nature in appreciable quantities, most actinides are rare. Most do not occur in nature, and of those that do, only thorium and uranium do so in more than trace quantities. The most abundant or easily synthesized actinides are uranium and thorium, followed by plutonium, americium, actinium, protactinium, neptunium, and curium.<ref>Myasoedov, p. 7</ref>

The existence of transuranium elements was suggested in 1934 by [[Enrico Fermi]], based on his experiments.<ref>{{cite journal|title=Possible Production of Elements of Atomic Number Higher than 92|journal=Nature|author= E. Fermi|bibcode=1934Natur.133..898F|year=1934|volume=133|pages=898–899|doi=10.1038/133898a0|issue=3372|doi-access=free}}</ref><ref>{{cite book|first1=Jagdish |last1=Mehra |first2=Helmut |last2=Rechenberg |author-link1=Jagdish Mehra|author-link2=Helmut Rechenberg|title=The historical development of quantum theory|url=https://books.google.com/books?id=kn6mb0ltm0UC&pg=PA966|year=2001|publisher=Springer|isbn=978-0-387-95086-0|page=966}}</ref> However, even though four actinides were known by that time, it was not yet understood that they formed a family similar to lanthanides. The prevailing view that dominated early research into transuranics was that they were regular elements in the 7th period, with thorium, protactinium and uranium corresponding to 6th-period [[hafnium]], [[tantalum]] and [[tungsten]], respectively. Synthesis of transuranics gradually undermined this point of view. By 1944, an observation that curium failed to exhibit oxidation states above 4 (whereas its supposed 6th period homolog, [[platinum]], can reach oxidation state of 6) prompted [[Glenn Seaborg]] to formulate an "[[actinide concept|actinide hypothesis]]". Studies of known actinides and discoveries of further transuranic elements provided more data in support of this position, but the phrase "actinide hypothesis" (the implication being that a "hypothesis" is something that has not been decisively proven) remained in active use by scientists through the late 1950s.<ref>{{cite book|title=Handbook on the Physics and Chemistry of Rare Earths|volume=18 – Lanthanides/Actinides: Chemistry|editor1=K.A. Gschneidner Jr., L|editor2=Eyring, G.R. Choppin|editor3=G.H. Landet|year=1994|publisher=Elsevier|chapter=118 – Origin of the actinide concept|author=Seaborg, G. T.|pages=4–6, 10–14}}</ref><ref>{{cite journal|doi=10.1021/ed036p340|title=The first isolations of the transuranium elements: A historical survey|year=1959|last1=Wallmann|first1=J. C.|journal=Journal of Chemical Education|volume=36|issue=7|page=340|bibcode = 1959JChEd..36..340W |url=http://www.escholarship.org/uc/item/7jx8p5z6}}</ref>

At present, there are two major methods of producing [[isotope]]s of transplutonium elements: (1) irradiation of the lighter elements with [[neutron]]s; (2) irradiation with accelerated charged particles. The first method is more important for applications, as only neutron irradiation using nuclear reactors allows the production of sizeable amounts of synthetic actinides; however, it is limited to relatively light elements. The advantage of the second method is that elements heavier than plutonium, as well as neutron-deficient isotopes, can be obtained, which are not formed during neutron irradiation.<ref>Myasoedov, p. 9</ref>

In 1962–1966, there were attempts in the United States to produce transplutonium isotopes using a series of six [[Underground nuclear testing|underground nuclear explosions]]. Small samples of rock were extracted from the blast area immediately after the test to study the explosion products, but no isotopes with [[mass number]] greater than 257 could be detected, despite predictions that such isotopes would have relatively long [[half-life|half-lives]] of [[Alpha decay|α-decay]]. This non-observation was attributed to [[spontaneous fission]] owing to the large speed of the products and to other decay channels, such as neutron emission and [[nuclear fission]].<ref>Myasoedov, p. 14</ref>

=== From actinium to uranium ===
[[File:Enrico Fermi 1943-49.jpg|thumb|left|[[Enrico Fermi]] suggested the existence of transuranium elements in 1934.]]

[[Uranium]] and [[thorium]] were the first actinides [[discovery of the chemical elements|discovered]]. Uranium was identified in 1789 by the German chemist [[Martin Heinrich Klaproth]] in [[Uraninite|pitchblende]] ore. He named it after the planet [[Uranus (planet)|Uranus]],<ref name=g1250 /> which had been discovered eight years earlier. Klaproth was able to precipitate a yellow compound (likely [[sodium diuranate]]) by dissolving [[pitchblende]] in [[nitric acid]] and neutralizing the solution with [[sodium hydroxide]]. He then reduced the obtained yellow powder with charcoal, and extracted a black substance that he mistook for metal.<ref>{{cite journal|title=Chemische Untersuchung des Uranits, einer neuentdeckten metallischen Substanz|author-link= Martin Heinrich Klaproth|author=Martin Heinrich Klaproth|url=https://books.google.com/books?id=YxQ_AAAAcAAJ&pg=PA387|journal=Chemische Annalen|volume=2|year=1789|pages=387–403}}</ref> Sixty years later, the French scientist [[Eugène-Melchior Péligot]] identified it as uranium oxide. He also isolated the first sample of uranium metal by heating [[uranium tetrachloride]] with metallic [[potassium]].<ref>{{cite journal| title=Recherches Sur L'Uranium|author=E.-M. Péligot|journal=[[Annales de chimie et de physique]]|volume=5|issue=5|year=1842|pages=5–47|url=http://gallica.bnf.fr/ark:/12148/bpt6k34746s/f4.table}}</ref> The [[atomic mass]] of uranium was then calculated as 120, but [[Dmitri Mendeleev]] in 1872 corrected it to 240 using his periodicity laws. This value was confirmed experimentally in 1882 by K. Zimmerman.<ref>{{cite book|doi=10.1007/1-4020-3598-5_5|author=Ingmar Grenthe|chapter=Uranium|title=The Chemistry of the Actinide and Transactinide Elements|pages=253–698|year=2006|isbn=978-1-4020-3555-5}}</ref><ref>K. Zimmerman, Ann., 213, 290 (1882); 216, 1 (1883); Ber. 15 (1882) 849</ref>

[[Thorium oxide]] was discovered by [[Friedrich Wöhler]] in the mineral [[thorianite]], which was found in Norway (1827).<ref>Golub, p. 214</ref> [[Jöns Jacob Berzelius]] characterized this material in more detail in 1828. By reduction of [[thorium tetrachloride]] with potassium, he isolated the metal and named it thorium after the [[norse mythology|Norse god]] of thunder and lightning [[Thor]].<ref>{{cite journal|author=Berzelius, J. J.|year=1829|url=http://gallica.bnf.fr/ark:/12148/bpt6k151010.pleinepage.r=Annalen+der+Physic.f395.langFR|title=Untersuchung eines neues Minerals und einer darin erhalten zuvor unbekannten Erde (Investigation of a new mineral and of a previously unknown earth contained therein)|journal=Annalen der Physik und Chemie|volume=16|pages=385–415|doi=10.1002/andp.18290920702|issue=7|bibcode=1829AnP....92..385B}} (modern citation: ''Annalen der Physik'', vol. 92, no. 7, pp. 385–415)</ref><ref>{{cite journal|author=Berzelius, J. J.|year=1829|title=Undersökning af ett nytt mineral (Thorit), som innehåller en förut obekant jord" (Investigation of a new mineral (thorite), as contained in a previously unknown earth)|journal=Kungliga Svenska Vetenskaps Akademiens Handlingar (Transactions of the Royal Swedish Science Academy)|url=http://ia800507.us.archive.org/30/items/kungligasvenska1182kung_2/kungligasvenska1182kung_2.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://ia800507.us.archive.org/30/items/kungligasvenska1182kung_2/kungligasvenska1182kung_2.pdf |archive-date=2022-10-09 |url-status=live|pages=1–30}}</ref> The same isolation method was later used by Péligot for uranium.<ref name=g1250 />

[[Actinium]] was discovered in 1899 by [[André-Louis Debierne]], an assistant of [[Marie Curie]], in the pitchblende waste left after removal of radium and polonium. He described the substance (in 1899) as similar to [[titanium]]<ref>{{cite journal|title=Sur un nouvelle matière radio-active|author=André-Louis Debierne|journal=Comptes Rendus|volume=129|pages=593–595|year=1899|url=http://gallica.bnf.fr/ark:/12148/bpt6k3085b/f593.table|language=fr}}</ref> and (in 1900) as similar to thorium.<ref>{{cite journal|title=Sur un nouvelle matière radio-actif – l'actinium|author=André-Louis Debierne|journal=Comptes Rendus|volume=130|pages=906–908|year=1900–1901|url=http://gallica.bnf.fr/ark:/12148/bpt6k3086n/f906.table|language=fr}}</ref> The discovery of actinium by Debierne was however questioned in 1971<ref>{{cite journal|title=The Discovery of Actinium|author=H. W. Kirby|journal=Isis|volume=62|issue=3|pages=290–308|year=1971|doi=10.1086/350760|jstor=229943|s2cid=144651011 }}</ref> and 2000,<ref>{{cite journal|title=The centenary of a controversial discovery: actinium|author=J. P. Adloff|journal=Radiochim. Acta|volume=88|pages=123–128|year=2000|doi=10.1524/ract.2000.88.3-4.123|issue=3–4_2000|s2cid=94016074 }}</ref> arguing that Debierne's publications in 1904 contradicted his earlier work of 1899–1900. This view instead credits the 1902 work of [[Friedrich Oskar Giesel]], who discovered a radioactive element named ''emanium'' that behaved similarly to lanthanum. The name actinium comes from the {{langx|grc|ακτίς, ακτίνος}} {{transliteration|grc|italic=no|(aktis, aktinos)}}, meaning beam or ray. This metal was discovered not by its own radiation but by the radiation of the daughter products.<ref>Golub, p. 213</ref><ref name="Himiya aktiniya">{{cite book|author1=Z. K. Karalova|author2=B. Myasoedov|title=Actinium|place=Moscow|publisher=[[Nauka (publisher)|Nauka]]|year=1982|series=Analytical chemistry items}}</ref> Owing to the close similarity of actinium and lanthanum and low abundance, pure actinium could only be produced in 1950. The term actinide was probably introduced by [[Victor Goldschmidt]] in 1937.<ref>{{cite journal|doi=10.1021/ed029p581.2|title=Letters|year=1952|last1=Hakala|first1=Reino W.|journal=Journal of Chemical Education|volume=29|issue=11|page=581|bibcode=1952JChEd..29..581H|doi-access=free}}</ref><ref>{{cite journal|doi=10.1007/s00897970143a|title=Victor Moritz Goldschmidt (1888–1947): A Tribute to the Founder of Modern Geochemistry on the Fiftieth Anniversary of His Death|year=1997|author=George B. Kauffman|author-link= George B. Kauffman|journal=The Chemical Educator|volume=2|issue=5|pages=1–26|s2cid=101664962 }}</ref><!-- He called them thorides in 1937-->

[[Protactinium]] was possibly isolated in 1900 by [[William Crookes]].<ref>{{cite book|title=Nature's Building Blocks: An A-Z Guide to the Elements|author=John Emsley|publisher=Oxford University Press|location=Oxford, England|isbn=978-0-19-850340-8|chapter=Protactinium|pages= 347–349|chapter-url=https://books.google.com/books?id=Yhi5X7OwuGkC|year=2001|url=https://archive.org/details/naturesbuildingb0000emsl/page/347}}</ref> It was first identified in 1913, when [[Kasimir Fajans]] and [[Oswald Helmuth Göhring]] encountered the short-lived isotope <sup>234m</sup>Pa (half-life 1.17 minutes) during their studies of the [[Decay chain#Uranium series|<sup>238</sup>U decay chain]]. They named the new element ''brevium'' (from Latin ''brevis'' meaning brief);<ref name=fajans>{{cite journal|author1=K. Fajans|author2=O. Gohring|title=Über die komplexe Natur des Ur X|journal=Naturwissenschaften|year=1913|volume=1|page=339|url=http://www.digizeitschriften.de/no_cache/home/jkdigitools/loader/?tx_jkDigiTools_pi1%5BIDDOC%5D=201162&tx_jkDigiTools_pi1%5Bpp%5D=425 |doi = 10.1007/BF01495360 |issue = 14|bibcode = 1913NW......1..339F |s2cid=40667401 }}</ref><ref>{{cite journal|author1=K. Fajans|author2=O. Gohring|title=Über das Uran X<sub>2</sub>-das neue Element der Uranreihe|journal=Physikalische Zeitschrift|year=1913|volume=14|pages=877–84}}</ref> the name was changed to ''protoactinium'' (from [[Greek language|Greek]] πρῶτος + ἀκτίς meaning "first beam element") in 1918 when two groups of scientists, led by the Austrian [[Lise Meitner]] and [[Otto Hahn]] of Germany and [[Frederick Soddy]] and [[John Arnold Cranston]] of Great Britain, independently discovered the much longer-lived <sup>231</sup>Pa. The name was shortened to ''protactinium'' in 1949. This element was little characterized until 1960, when [[Alfred Maddock]] and his co-workers in the U.K. isolated 130&nbsp;grams of protactinium from 60 tonnes of waste left after extraction of uranium from its ore.<ref name=g1251>Greenwood, p. 1251</ref>

=== Neptunium and above ===
Neptunium (named for the planet [[Neptune]], the next [[planet]] out from Uranus, after which uranium was named) was discovered by [[Edwin McMillan]] and [[Philip H. Abelson]] in 1940 in [[Berkeley, California]].<ref>{{cite journal|doi=10.1103/PhysRev.57.1185.2|title=Radioactive Element 93|year=1940|author=Edwin McMillan|journal=Physical Review|volume=57|pages=1185–1186|last2=Abelson|first2=Philip|issue=12|bibcode=1940PhRv...57.1185M|doi-access=free}}</ref> They produced the <sup>239</sup>Np isotope (half-life 2.4 days) by bombarding uranium with slow [[neutron]]s.<ref name=g1251 /> It was the first [[transuranium element]] produced synthetically.<ref name="Himiya neptuniya">{{cite book|title=Analytical chemistry of neptunium|editor=V.A. Mikhailov|place=Moscow|publisher=Nauka|year=1971}}</ref>

[[File:Glenn Seaborg - 1964.jpg|thumb|[[Glenn T. Seaborg]] and his group at the [[University of California at Berkeley]] synthesized Pu, Am, Cm, Bk, Cf, Es, Fm, Md, No and element 106, which was later named [[seaborgium]] in his honor while he was still living. They also synthesized more than a hundred actinide isotopes.]]

Transuranium elements do not occur in sizeable quantities in nature and are commonly synthesized via [[nuclear reaction]]s conducted with nuclear reactors. For example, under irradiation with reactor neutrons, [[uranium-238]] partially converts to [[plutonium-239]]:
: <math chem>\ce{{^{238}_{92}U} + {}^{1}_{0}n -> {}^{239}_{92}U ->[\beta^-] [23.5\ \ce{min}] {}^{239}_{93}Np ->[\beta^-] [2.3\ \ce{days}] {}^{239}_{94}Pu} \left( \ce{->[\alpha] [2.4\cdot 10^4\ \ce{years}]} \right) \ce{{^{235}_{92}U}}</math>

This synthesis reaction was used by Fermi and his collaborators in their design of the reactors located at the [[Hanford Site]], which produced significant amounts of plutonium-239 for the nuclear weapons of the [[Manhattan Project]] and the United States' post-war nuclear arsenal.<ref>{{cite book|last=Hanford Cultural Resources Program, US Department of Energy|title=Hanford Site Historic District: History of the Plutonium Production Facilities, 1943–1990|publisher=Battelle Press|year=2002|location=Columbus OH|isbn=978-1-57477-133-6|pages=1.22–1.27|url=http://www.osti.gov/scitech/servlets/purl/807939|doi=10.2172/807939 |osti=807939 }}</ref>

Actinides with the highest mass numbers are synthesized by bombarding uranium, plutonium, curium and californium with [[ion]]s of nitrogen, oxygen, carbon, neon or boron in a [[particle accelerator]]. Thus [[nobelium]] was produced by bombarding uranium-238 with [[neon-22]] as
: <chem>_{92}^{238}U + _{10}^{22}Ne -> _{102}^{256}No + 4_0^1n</chem>.

The first isotopes of transplutonium elements, [[americium-241]] and [[curium-242]], were synthesized in 1944 by [[Glenn T. Seaborg]], [[Ralph A. James]] and [[Albert Ghiorso]].<ref>{{cite book|title=The New Chemistry: A Showcase for Modern Chemistry and Its Applications|author=Nina Hall|publisher=Cambridge University Press|year=2000|pages=8–9|isbn=978-0-521-45224-3|url=https://archive.org/details/newchemistry00hall|url-access=registration}}</ref> Curium-242 was obtained by bombarding plutonium-239 with 32-MeV α-particles:
: <chem>_{94}^{239}Pu + _2^4He -> _{96}^{242}Cm + _0^1n</chem>.

The americium-241 and curium-242 isotopes also were produced by irradiating plutonium in a nuclear reactor. The latter element was named after [[Marie Curie]] and her husband [[Pierre Curie|Pierre]] who are noted for discovering [[radium]] and for their work in [[radioactivity]].<ref>Myasoedov, p. 8</ref>

Bombarding curium-242 with α-particles resulted in an isotope of californium [[Californium-245|<sup>245</sup>Cf]] in 1950, and a similar procedure yielded [[berkelium-243]] from americium-241 in 1949.<ref>{{cite journal|first1=S. G.|last1=Thompson|first2=A.|last2=Ghiorso|author-link2=Albert Ghiorso|first3=G. T.|last3=Seaborg|author-link3=Glenn T. Seaborg|title=Element 97|journal=Phys. Rev.|year=1950|volume=77|issue=6|pages=838–839|doi=10.1103/PhysRev.77.838.2|bibcode=1950PhRv...77..838T |doi-access=free}}</ref> The new elements were named after [[Berkeley, California]], by analogy with its [[lanthanide]] [[Homologous series|homologue]] [[terbium]], which was named after the village of [[Ytterby]] in Sweden.<ref>{{cite journal|first1 = S. G.|last1=Thompson|first2=A.|last2=Ghiorso|author-link2=Albert Ghiorso|first3=G. T.|last3=Seaborg|author-link3=Glenn T. Seaborg|title=The New Element Berkelium (Atomic Number 97)|journal=Phys. Rev.|year=1950|volume=80|pages=781–789|doi=10.1103/PhysRev.80.781|issue=5|bibcode=1950PhRv...80..781T|url=https://digital.library.unt.edu/ark:/67531/metadc894817/}}</ref>

In 1945, B. B. Cunningham obtained the first bulk chemical compound of a transplutonium element, namely [[Americium(III) hydroxide|americium hydroxide]].<ref>Wallace W. Schulz (1976) [http://www.osti.gov/bridge/purl.cover.jsp;jsessionid=99C379B4BBA56BB186AAD989333D2B5E?purl=/7232133-fyKvqE/ The Chemistry of Americium], U.S. Department of Commerce, p. 1</ref> Over the few years, milligram quantities of americium and microgram amounts of curium were accumulated that allowed production of isotopes of berkelium<ref>{{cite journal|last1=Thompson|first1=S.|last2=Ghiorso|first2=A.|last3=Seaborg|first3=G.|title=Element 97|journal=Physical Review|volume=77|pages=838–839|year=1950|doi=10.1103/PhysRev.77.838.2|issue=6|bibcode=1950PhRv...77..838T|doi-access=free}}</ref><ref>{{cite journal|last1=Thompson|first1=S.|last2=Ghiorso|first2=A.|last3=Seaborg|first3=G.|title=The New Element Berkelium (Atomic Number 97)|journal=Physical Review|volume=80|pages=781–789|year=1950|doi=10.1103/PhysRev.80.781|issue=5|bibcode=1950PhRv...80..781T|url=https://digital.library.unt.edu/ark:/67531/metadc894817/}}</ref> and californium.<ref>{{cite journal|author1=S. G. Thompson|author2=K. Street Jr.|author3=A. Ghiorso|author4=G. T. Seaborg|title=Element 98|journal=[[Physical Review]]|year=1950|volume=78|pages=298–299|doi=10.1103/PhysRev.78.298.2|url=http://repositories.cdlib.org/cgi/viewcontent.cgi?article=7072&context=lbnl|issue=3|bibcode=1950PhRv...78..298T|doi-access=free}}</ref><ref>{{cite journal|author1=S. G. Thompson|author2=K. Street Jr.|author3=A. Ghiorso|author4=G. T. Seaborg| title=The New Element Californium (Atomic Number 98)|journal=Physical Review|year=1950|volume=80|pages=790–796|doi=10.1103/PhysRev.80.790|url=http://www.osti.gov/accomplishments/documents/fullText/ACC0050.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.osti.gov/accomplishments/documents/fullText/ACC0050.pdf |archive-date=2022-10-09 |url-status=live|issue=5|bibcode=1950PhRv...80..790T}}</ref><ref>{{cite journal|author1=K. Street Jr.|author2=S. G. Thompson|author3=G. T. Seaborg|title=Chemical Properties of Californium|journal=[[J. Am. Chem. Soc.]]|year=1950|volume=72|pages=4832–4835|doi=10.1021/ja01166a528|url=http://handle.dtic.mil/100.2/ADA319899|archive-url=http://arquivo.pt/wayback/20160515073629/http://handle.dtic.mil/100.2/ADA319899|url-status=dead|archive-date=15 May 2016|issue=10|bibcode=1950JAChS..72R4832S |hdl=2027/mdp.39015086449173|access-date=23 October 2010}}</ref> Sizeable amounts of these elements were produced in 1958,<ref>S. G. Thompson and B. B. Cunningham (1958) [https://escholarship.org/uc/item/1wh7c44z "First Macroscopic Observations of the Chemical Properties of Berkelium and Californium"], supplement to Paper P/825 presented at the Second Intl. Conf., Peaceful Uses Atomic Energy, Geneva</ref> and the first californium compound (0.3&nbsp;μg of CfOCl) was obtained in 1960 by B. B. Cunningham and J. C. Wallmann.<ref>Darleane C. Hoffman, Albert Ghiorso, Glenn Theodore Seaborg (2000) ''The transuranium people: the inside story'', Imperial College Press, {{ISBN|1-86094-087-0}}, pp. 141–142</ref>

Einsteinium and fermium were identified in 1952–1953 in the fallout from the "[[Ivy Mike]]" nuclear test (1 November 1952), the first successful test of a hydrogen bomb. Instantaneous exposure of uranium-238 to a large neutron flux resulting from the explosion produced heavy isotopes of uranium, which underwent a series of [[beta decay]]s to nuclides such as [[einsteinium-253]] and [[fermium-255]]. The discovery of the new elements and the new data on neutron capture were initially kept secret on the orders of the US&nbsp;military until 1955 due to [[Cold War]] tensions.<ref name="PR1955" /><ref name="PhysRev.99.1048">{{cite journal|title=New Elements Einsteinium and Fermium, Atomic Numbers 99 and 100|author1=A. Ghiorso|author2=S. G. Thompson|author3=G. H. Higgins|author4=G. T. Seaborg|author5=M. H. Studier|author6=P. R. Fields|author7=S. M. Fried|author8=H. Diamond|author9=J. F. Mech|author10=G. L. Pyle|author11=J. R. Huizenga|author12=A. Hirsch|author13=W. M. Manning|author14=C. I. Browne|author15=H. L. Smith|author16=R. W. Spence|journal=Phys. Rev.|volume=99|issue=3|doi=10.1103/PhysRev.99.1048|pages=1048–1049|year=1955|bibcode=1955PhRv...99.1048G|url=https://digital.library.unt.edu/ark:/67531/metadc889467/|doi-access=free}}</ref> Nevertheless, the Berkeley team were able to prepare einsteinium and fermium by civilian means, through the neutron bombardment of plutonium-239, and published this work in 1954 with the disclaimer that it was not the first studies that had been carried out on those elements.<ref>{{cite journal|journal=Physical Review|volume=93|year=1954|title=Transcurium Isotopes Produced in the Neutron Irradiation of Plutonium|author1=S. Thompson|author2=A. Ghiorso|author3=B. G. Harvey|author4=G. R. Choppin|doi=10.1103/PhysRev.93.908|page=908|issue=4|bibcode=1954PhRv...93..908T|url=https://digital.library.unt.edu/ark:/67531/metadc1016991/|doi-access=free}}</ref><ref>{{Cite journal|author1=G. R. Choppin|author2=S. G. Thompson|author3=A. Ghiorso|author4=B. G. Harvey|title=Nuclear Properties of Some Isotopes of Californium, Elements 99 and 100|journal=Physical Review|volume=94|issue=4|pages=1080–1081|year=1954|doi=10.1103/PhysRev.94.1080|bibcode=1954PhRv...94.1080C|doi-access=free}}</ref> The "Ivy Mike" studies were declassified and published in 1955.<ref name="PhysRev.99.1048" /> The first significant (submicrogram) amounts of einsteinium were produced in 1961 by Cunningham and colleagues, but this has not been done for fermium yet.<ref>{{cite journal|author=Albert Ghiorso|author-link=Albert Ghiorso|year=2003|title=Einsteinium and Fermium|journal=Chemical and Engineering News|url=http://pubs.acs.org/cen/80th/einsteiniumfermium.html|volume=81|issue=36}}</ref>

The first isotope of mendelevium, [[mendelevium-256|<sup>256</sup>Md]] (half-life 87 min), was synthesized by Albert Ghiorso, Glenn T. Seaborg, [[Gregory Robert Choppin]], Bernard G. Harvey and [[Stanley Gerald Thompson]] when they bombarded an <sup>253</sup>Es target with [[alpha particle]]s in the 60-inch [[cyclotron]] of [[Berkeley Radiation Laboratory]]; this was the first isotope of any element to be synthesized one atom at a time.<ref>{{cite book|doi=10.1103/PhysRev.98.1518|url=https://books.google.com/books?id=e53sNAOXrdMC&pg=PA101|isbn=978-981-02-1440-1|title=New Element Mendelevium, Atomic Number 101|year=1955|author1=A. Ghiorso |author2=B. Harvey |author3=G. Choppin|author4=S. Thompson|author5=G. Seaborg|journal=Physical Review|volume=98|pages=1518–1519|issue=5|bibcode=1955PhRv...98.1518G}}</ref>

There were several attempts to obtain isotopes of nobelium by Swedish (1957) and American (1958) groups, but the first reliable result was the synthesis of [[Nobelium-256|<sup>256</sup>No]] by the Russian group of [[Georgy Flyorov]] in 1965, as acknowledged by the [[IUPAC]] in 1992. In their experiments, Flyorov et al. bombarded uranium-238 with neon-22.<ref name=g1252 />

In 1961, Ghiorso et al. obtained the first isotope of lawrencium by irradiating californium (mostly [[californium-252]]) with [[boron-10]] and [[boron-11]] ions.<ref name=g1252 /> The [[mass number]] of this isotope was not clearly established (possibly 258 or 259) at the time. In 1965, [[Lawrencium-256|<sup>256</sup>Lr]] was synthesized by Flyorov et al. from [[Americium-243|<sup>243</sup>Am]] and [[oxygen-18|<sup>18</sup>O]]. Thus IUPAC recognized the nuclear physics teams at Dubna and Berkeley as the co-discoverers of lawrencium.

== Isotopes ==
{| class="wikitable collapsible collapsed" style="text-align:center;"
!+ colspan=7 | Nuclear properties of isotopes of the most important transplutonium isotopes{{NUBASE2020|ref}}<ref name="tablitsa MAGATE" /><ref>Myasoedov, pp. 19–21</ref>
|-
! rowspan="2"| Isotope
! rowspan="2"| [[Half-life]]
! rowspan="2"| Probability of <br />[[spontaneous fission|spontaneous<br />fission]] in %
! colspan="2"| Emission energy<br />(MeV) (yield in %)
! colspan="2"| Specific activity (Bq/kg)<ref group=notes>Specific activity is calculated by given in the table half-lives and the probability of spontaneous fission</ref> of
|-
! α
! γ
! α, β-particles
! fission
|-
|[[Americium-241|<sup>241</sup>Am]] || 432.2(7) y || 4.3(18){{e|−10}} || 5.485 (84.8) <br />5.442 (13.1)<br />5.388 (1.66) || 0.059 (35.9)<br />0.026 (2.27) || 1.27{{e|14}} || 546.1
|-
|[[Americium-243|<sup>243</sup>Am]] || 7.37(4){{e|3}} y || 3.7(2){{e|−9}} || 5.275 (87.1)<br />5.233 (11.2)<br />5.181 (1.36) || 0.074 (67.2)<br /> 0.043 (5.9) || 7.39{{e|12}} || 273.3
|-
|[[Curium-242|<sup>242</sup>Cm]] || 162.8(2) d || 6.2(3){{e|−6}} || 6.069 (25.92)<br />6.112 (74.08) || 0.044 (0.04)<br />0.102 (4{{e|−3}}) || 1.23{{e|17}} || 7.6{{e|9}}
|-
|[[Curium-244|<sup>244</sup>Cm]] || 18.10(2) y || 1.37(3){{e|−4}} || 5.762 (23.6)<br />5.804 (76.4) || 0.043 (0.02)<br />0.100 (1.5{{e|−3}}) || 2.96{{e|15}} || 4.1{{e|9}}
|-
|[[Curium-245|<sup>245</sup>Cm]] || 8.5(1){{e|3}} y || 6.1(9){{e|−7}} || 5.529 (0.58)<br />5.488 (0.83)<br />5.361 (93.2) || 0.175 (9.88)<br />0.133 (2.83) || 6.35{{e|12}} || 3.9{{e|4}}
|-
|[[Curium-246|<sup>246</sup>Cm]] || 4.76(4){{e|3}} y || 0.02615(7) || 5.343 (17.8)<br />5.386 (82.2) || 0.045 (19) || 1.13{{e|13}} || 2.95{{e|9}}
|-
|[[Curium-247|<sup>247</sup>Cm]] || 1.56(5){{e|7}} y || — || 5.267 (13.8)<br />5.212 (5.7)<br />5.147 (1.2) || 0.402 (72)<br />0.278 (3.4) || 3.43{{e|9}} ||—
|-
|[[Curium-248|<sup>248</sup>Cm]] || 3.48(6){{e|5}} y || 8.39(16) || 5.034 (16.52)<br />5.078 (75) ||—|| 1.40{{e|11}} || 1.29{{e|10}}
|-
|[[Berkelium-249|<sup>249</sup>Bk]] || 330(4) d || 4.7(2){{e|−8}} || 5.406 (1{{e|−3}})<br />5.378 (2.6{{e|−4}}) || 0.32 (5.8{{e|−5}}) || 5.88{{e|16}} || 2.76{{e|7}}
|-
|[[Californium-249|<sup>249</sup>Cf]] || 351(2) y || 5.0(4){{e|−7}} || 6.193 (2.46)<br />6.139 (1.33)<br />5.946 (3.33) || 0.388 (66)<br />0.333 (14.6) || 1.51{{e|14}} || 7.57{{e|5}}
|-
|[[Californium-250|<sup>250</sup>Cf]] || 13.08(9) y || 0.077(3) || 5.988 (14.99)<br />6.030 (84.6) || 0.043 || 4.04{{e|15}} || 3.11{{e|12}}
|-
|[[Californium-251|<sup>251</sup>Cf]] || 900(40) y || ? || 6.078 (2.6)<br />5.567 (0.9)<br />5.569 (0.9) || 0.177 (17.3)<br />0.227 (6.8) || 5.86{{e|13}} ||—
|-
|[[Californium-252|<sup>252</sup>Cf]] || 2.645(8) y || 3.092(8) || 6.075 (15.2)<br />6.118 (81.6) || 0.042 (1.4{{e|−2}})<br />0.100 (1.3{{e|−2}}) || 1.92{{e|16}} || 6.14{{e|14}}
|-
|[[Californium-254|<sup>254</sup>Cf]] || 60.5(2) d || ≈100 || 5.834 (0.26)<br />5.792 (5.3{{e|−2}}) ||—|| 9.75{{e|14}} || 3.13{{e|17}}
|-
|[[Einsteinium-253|<sup>253</sup>Es]] || 20.47(3) d || 8.7(3){{e|−6}} || 6.540 (0.85)<br />6.552 (0.71)<br />6.590 (6.6) || 0.387 (0.05)<br />0.429 (8{{e|−3}}) || 9.33{{e|17}} || 8.12{{e|10}}
|-
|[[Einsteinium-254|<sup>254</sup>Es]] || 275.7(5) d || < 3{{e|−6}} || 6.347 (0.75)<br />6.358 (2.6)<br />6.415 (1.8) || 0.042 (100)<br />0.034 (30) || 6.9{{e|16}} ||—
|-
|[[Einsteinium-255|<sup>255</sup>Es]] || 39.8(12) d || 0.0041(2) || 6.267 (0.78)<br />6.401 (7) ||—|| 4.38{{e|17}}(β)<br />3.81{{e|16}}(α) || 1.95{{e|13}}
|-
|[[Fermium-255|<sup>255</sup>Fm]] || 20.07(7) h || 2.4(10){{e|−5}} || 7.022 (93.4)<br />6.963 (5.04)<br />6.892 (0.62) || 0.00057 (19.1)<br />0.081 (1) || 2.27{{e|19}} || 5.44{{e|12}}
|-
|[[Fermium-256|<sup>256</sup>Fm]] || 157.6(13) min || 91.9(3) || 6.872 (1.2)<br />6.917 (6.9) ||—||1.58{{e|20}} || 1.4{{e|19}}
|-
|[[Fermium-257|<sup>257</sup>Fm]] || 100.5(2) d || 0.210(4) || 6.752 (0.58)<br />6.695 (3.39)<br />6.622 (0.6) || 0.241 (11)<br />0.179 (8.7) || 1.87{{e|17}} || 3.93{{e|14}}
|-
|[[Mendelevium-256|<sup>256</sup>Md]] || 77(2) min ||—|| 7.142 (1.84)<br />7.206 (5.9) ||—||3.53{{e|20}} ||—
|-
|[[Mendelevium-257|<sup>257</sup>Md]] || 5.52(5) h ||—|| 7.074 (14) || 0.371 (11.7)<br />0.325 (2.5) ||8.17{{e|19}}||—
|-
|[[Mendelevium-258|<sup>258</sup>Md]] || 51.5(3) d ||—|| 6.73||—||3.64{{e|17}} ||—
|-
|[[Nobelium-255|<sup>255</sup>No]] || 3.1(2) min ||—|| 8.312 (1.16)<br />8.266 (2.6)<br />8.121 (27.8) || 0.187 (3.4) || 8.78{{e|21}} ||—
|-
|[[Nobelium-259|<sup>259</sup>No]] || 58(5) min ||—|| 7.455 (9.8)<br />7.500 (29.3)<br />7.533 (17.3) ||—||4.63{{e|20}}||—
|-
|[[Lawrencium-256|<sup>256</sup>Lr]] || 27(3) s ||< 0.03|| 8.319 (5.4)<br />8.390 (16)<br />8.430 (33) ||—|| 5.96{{e|22}} ||—
|-
|[[Lawrencium-257|<sup>257</sup>Lr]] || 646(25) ms ||—|| 8.796 (18)<br />8.861 (82) ||—||1.54{{e|24}}||—
|}

[[File:Isotopes and half-life.svg|thumb|upright=1.5|Actinides have 89–103 protons and usually 117–159 neutrons.]]
Thirty-four [[isotopes of actinium]] and eight excited [[nuclear isomerism|isomeric states]] of some of its [[nuclide]]s are known, ranging in mass number from 203 to 236.{{NUBASE2020|ref}} Three isotopes, [[Actinium-225|<sup>225</sup>Ac]], [[Actinium-227|<sup>227</sup>Ac]] and [[Actinium-228|<sup>228</sup>Ac]], were found in nature and the others were produced in the laboratory; only the three natural isotopes are used in applications. Actinium-225 is a member of the radioactive [[neptunium series]];<ref name=g1254>Greenwood, p. 1254</ref> it was first discovered in 1947 as a decay product of [[uranium-233]] and it is an α-emitter with a half-life of 10 days. Actinium-225 is less available than actinium-228, but is more promising in radiotracer applications.<ref name="Himiya aktiniya" /> Actinium-227 (half-life 21.77 years) occurs in all uranium ores, but in small quantities. One gram of uranium (in radioactive equilibrium) contains only 2{{e|-10}} gram of <sup>227</sup>Ac.<ref name="Himiya aktiniya" />{{NUBASE2020|ref}} Actinium-228 is a member of the [[Radioactive series#Thorium series|radioactive thorium series]] formed by the decay of [[Radium-228|<sup>228</sup>Ra]];<ref name=g1254 /> it is a β<sup>−</sup> emitter with a half-life of 6.15 hours. In one tonne of thorium there is 5{{e|-8}} gram of <sup>228</sup>Ac. It was discovered by [[Otto Hahn]] in 1906.<ref name="Himiya aktiniya" />

There are 32 known [[isotopes of thorium]] ranging in mass number from 207 to 238.{{NUBASE2020|ref}} Of these, the longest-lived is <sup>232</sup>Th, whose half-life of {{val|1.4|e=10|u=years}} means that it still exists in nature as a [[primordial nuclide]]. The next longest-lived is <sup>230</sup>Th, an intermediate decay product of <sup>238</sup>U with a half-life of 75,400 years. Several other thorium isotopes have half-lives over a day; all of these are also transient in the decay chains of <sup>232</sup>Th, <sup>235</sup>U, and <sup>238</sup>U.

Twenty-nine [[isotopes of protactinium]] are known with mass numbers 211–239{{NUBASE2020|ref}} as well as three excited [[nuclear isomerism|isomeric states]]. Only [[protactinium-231|<sup>231</sup>Pa]] and [[protactinium-234|<sup>234</sup>Pa]] have been found in nature. All the isotopes have short lifetimes, except for protactinium-231 (half-life 32,760 years). The most important isotopes are <sup>231</sup>Pa and [[protactinium-233|<sup>233</sup>Pa]], which is an intermediate product in obtaining uranium-233 and is the most affordable among artificial isotopes of protactinium. <sup>233</sup>Pa has convenient half-life and energy of [[Gamma radiation|γ-radiation]], and thus was used in most studies of protactinium chemistry. Protactinium-233 is a [[Beta radiation|β-emitter]] with a half-life of 26.97 days.{{NUBASE2020|ref}}<ref name="Himiya protaktiniya" />

There are 27 known [[isotopes of uranium]], having mass numbers 215–242 (except 220).<ref name="tablitsa MAGATE" /> Three of them, [[Uranium-234|<sup>234</sup>U]], <sup>235</sup>U and <sup>238</sup>U, are present in appreciable quantities in nature. Among others, the most important is <sup>233</sup>U, which is a final product of transformation of [[Thorium-232|<sup>232</sup>Th]] irradiated by slow neutrons. <sup>233</sup>U has a much higher fission efficiency by low-energy (thermal) neutrons, compared e.g. with <sup>235</sup>U. Most uranium chemistry studies were carried out on uranium-238 owing to its long half-life of 4.4{{e|9}} years.<ref>{{cite book|author=I.P. Alimarin|title=Analytical chemistry of uranium|editor=A.P. Vinogradov|location=Moscow|publisher=Publisher USSR Academy of Sciences|year=1962}}</ref>

There are 25 [[isotopes of neptunium]] with mass numbers 219–244 (except 221);<ref name="tablitsa MAGATE" /> they are all highly radioactive. The most popular among scientists are long-lived <sup>237</sup>Np (t<sub>1/2</sub>&nbsp;= 2.20{{e|6}} years) and short-lived <sup>239</sup>Np, <sup>238</sup>Np (t<sub>1/2</sub>&nbsp;~ 2 days).<ref name="Himiya neptuniya" />

There are 21 known [[isotopes of plutonium]], having mass numbers 227–247.<ref name="tablitsa MAGATE" /> The most stable isotope of plutonium is <sup>244</sup>Pu with half-life of 8.13{{e|7}} years.{{NUBASE2020|ref}}

Eighteen [[isotopes of americium]] are known with mass numbers from 229 to 247 (with the exception of 231).<ref name="tablitsa MAGATE" /> The most important are <sup>241</sup>Am and <sup>243</sup>Am, which are alpha-emitters and also emit soft, but intense γ-rays; both of them can be obtained in an isotopically pure form. Chemical properties of americium were first studied with <sup>241</sup>Am, but later shifted to <sup>243</sup>Am, which is almost 20 times less radioactive. The disadvantage of <sup>243</sup>Am is production of the short-lived daughter isotope <sup>239</sup>Np, which has to be considered in the data analysis.<ref name=m18>Myasoedov, p. 18</ref>

Among 19 [[isotopes of curium]], ranging in mass number from 233 to 251,<ref name="tablitsa MAGATE" /> the most accessible are <sup>242</sup>Cm and <sup>244</sup>Cm; they are α-emitters, but with much shorter lifetime than the americium isotopes. These isotopes emit almost no γ-radiation, but undergo [[spontaneous fission]] with the associated emission of neutrons. More long-lived isotopes of curium (<sup>245–248</sup>Cm, all α-emitters) are formed as a mixture during neutron irradiation of plutonium or americium. Upon short irradiation, this mixture is dominated by <sup>246</sup>Cm, and then <sup>248</sup>Cm begins to accumulate. Both of these isotopes, especially <sup>248</sup>Cm, have a longer half-life (3.48{{e|5}} years) and are much more convenient for carrying out chemical research than <sup>242</sup>Cm and <sup>244</sup>Cm, but they also have a rather high rate of spontaneous fission. <sup>247</sup>Cm has the longest lifetime among isotopes of curium (1.56{{e|7}} years), but is not formed in large quantities because of the strong fission induced by thermal neutrons.

Seventeen [[isotopes of berkelium]] have been identified with mass numbers 233, 234, 236, 238, and 240–252.<ref name="tablitsa MAGATE" /> Only <sup>249</sup>Bk is available in large quantities; it has a relatively short half-life of 330 days and emits mostly soft [[Beta decay|β-particles]], which are inconvenient for detection. Its [[alpha radiation]] is rather weak (1.45{{e|-3}}% with respect to β-radiation), but is sometimes used to detect this isotope. <sup>247</sup>Bk is an alpha-emitter with a long half-life of 1,380 years, but it is hard to obtain in appreciable quantities; it is not formed upon neutron irradiation of plutonium because β-decay of curium isotopes with mass number below 248 is not known.<ref name=m18 /> (<sup>247</sup>Cm would actually release energy by β-decaying to <sup>247</sup>Bk, but this has never been seen.)

The 20 [[isotopes of californium]] with mass numbers 237–256 are formed in nuclear reactors;<ref name="tablitsa MAGATE" /> californium-253 is a β-emitter and the rest are α-emitters. The isotopes with even mass numbers (<sup>250</sup>Cf, <sup>252</sup>Cf and <sup>254</sup>Cf) have a high rate of spontaneous fission, especially <sup>254</sup>Cf of which 99.7% decays by spontaneous fission. Californium-249 has a relatively long half-life (352 years), weak spontaneous fission and strong γ-emission that facilitates its identification. <sup>249</sup>Cf is not formed in large quantities in a nuclear reactor because of the slow β-decay of the parent isotope <sup>249</sup>Bk and a large cross section of interaction with neutrons, but it can be accumulated in the isotopically pure form as the β-decay product of (pre-selected) <sup>249</sup>Bk. Californium produced by reactor-irradiation of plutonium mostly consists of <sup>250</sup>Cf and <sup>252</sup>Cf, the latter being predominant for large neutron fluences, and its study is hindered by the strong neutron radiation.<ref name=m22>Myasoedov, p. 22</ref>

{| class="wikitable"  style="float:right; text-align:center;"
|+ Properties of some transplutonium isotope pairs<ref>Myasoedov, p. 25</ref>
! Parent <br />isotope
! t<sub>1/2</sub>
! Daughter <br />isotope
! t<sub>1/2</sub>
! Time to establish <br />radioactive equilibrium
|-
| <sup>243</sup>Am|| 7370 years|| <sup>239</sup>Np|| 2.35 days|| 47.3 days
|-
| <sup>245</sup>Cm|| 8265 years|| <sup>241</sup>Pu|| 14 years|| 129 years
|-
| <sup>247</sup>Cm|| 1.64{{e|7}} years|| <sup>243</sup>Pu|| 4.95 hours|| 7.2 days
|-
| <sup>254</sup>Es|| 270 days|| <sup>250</sup>Bk|| 3.2 hours|| 35.2 hours
|-
| <sup>255</sup>Es|| 39.8 days|| <sup>255</sup>Fm|| 22 hours|| 5 days
|-
| <sup>257</sup>Fm|| 79 days|| <sup>253</sup>Cf|| 17.6 days|| 49 days
|}

Among the 18 known [[isotopes of einsteinium]] with mass numbers from 240 to 257,<ref name="tablitsa MAGATE">{{cite web|url=http://www-nds.iaea.org/relnsd/vcharthtml/VChartHTML.html|title=Table of nuclides, IAEA|access-date=7 July 2010}}</ref> the most affordable is <sup>253</sup>Es. It is an α-emitter with a half-life of 20.47 days, a relatively weak γ-emission and small spontaneous fission rate as compared with the isotopes of californium. Prolonged neutron irradiation also produces a long-lived isotope <sup>254</sup>Es (t<sub>1/2</sub>&nbsp;= 275.5 days).<ref name=m22 />

Twenty [[isotopes of fermium]] are known with mass numbers of 241–260. <sup>254</sup>Fm, <sup>255</sup>Fm and <sup>256</sup>Fm are [[Alpha radiation|α-emitters]] with a short half-life (hours), which can be isolated in significant amounts. <sup>257</sup>Fm (t<sub>1/2</sub> = 100 days) can accumulate upon prolonged and strong irradiation. All these isotopes are characterized by high rates of spontaneous fission.<ref name=m22 /><ref name="Tablitsa izotopov">{{cite web|url=http://elm.e-science.ru/|title=Table of elements, compounds, isotopes|language=ru|access-date=7 July 2010|archive-url=https://web.archive.org/web/20100712111619/http://elm.e-science.ru/|archive-date=12 July 2010|url-status=dead}}</ref>

Among the 17 known [[isotopes of mendelevium]] (mass numbers from 244 to 260),<ref name="tablitsa MAGATE" /> the most studied is <sup>256</sup>Md, which mainly decays through electron capture (α-radiation is ≈10%) with a half-life of 77 minutes. Another alpha emitter, <sup>258</sup>Md, has a half-life of 53 days. Both these isotopes are produced from rare einsteinium (<sup>253</sup>Es and <sup>255</sup>Es respectively), that therefore limits their availability.{{NUBASE2020|ref}}

Long-lived [[isotopes of nobelium]] and [[isotopes of lawrencium]] (and of heavier elements) have relatively short half-lives. For nobelium, 13 isotopes are known, with mass numbers 249–260 and 262. The chemical properties of nobelium and lawrencium were studied with <sup>255</sup>No (t<sub>1/2</sub>&nbsp;= 3&nbsp;min) and <sup>256</sup>Lr (t<sub>1/2</sub>&nbsp;= 35&nbsp;s). The longest-lived nobelium isotope, <sup>259</sup>No, has a half-life of approximately 1 hour.{{NUBASE2020|ref}} Lawrencium has 14 known isotopes with mass numbers 251–262, 264, and 266. The most stable of them is <sup>266</sup>Lr with a half-life of 11 hours.

Among all of these, the only isotopes that occur in sufficient quantities in nature to be detected in anything more than traces and have a measurable contribution to the atomic weights of the actinides are the primordial <sup>232</sup>Th, <sup>235</sup>U, and <sup>238</sup>U, and three long-lived decay products of natural uranium, <sup>230</sup>Th, <sup>231</sup>Pa, and <sup>234</sup>U. Natural thorium consists of 0.02(2)% <sup>230</sup>Th and 99.98(2)% <sup>232</sup>Th; natural protactinium consists of 100% <sup>231</sup>Pa; and natural uranium consists of 0.0054(5)% <sup>234</sup>U, 0.7204(6)% <sup>235</sup>U, and 99.2742(10)% <sup>238</sup>U.<ref>[http://www.ciaaw.org/atomic-weights.htm Standard Atomic Weights 2013]. [[Commission on Isotopic Abundances and Atomic Weights]]</ref>

== Formation in nuclear reactors ==
[[File:Actinide Buildup Chart 03a.png|thumb|upright=1.5|Table of nuclides: Buildup of actinides in a nuclear reactor, including radioactive decay]]
The figure ''buildup of actinides'' is a table of nuclides with the number of neutrons on the horizontal axis (isotopes) and the number of protons on the vertical axis (elements). The red dot divides the nuclides in two groups, so the figure is more compact. Each nuclide is represented by a square with the mass number of the element and its half-life.<ref>{{cite journal | last1=Soppera | first1=N. | last2=Bossant | first2=M. | last3=Dupont | first3=E. | title=JANIS 4: An Improved Version of the NEA Java-based Nuclear Data Information System | journal=Nuclear Data Sheets | publisher=Elsevier BV | volume=120 | year=2014 | doi=10.1016/j.nds.2014.07.071 | pages=294–296| bibcode=2014NDS...120..294S }}</ref> Naturally existing actinide isotopes (Th, U) are marked with a bold border, alpha emitters have a yellow colour, and beta emitters have a blue colour. Pink indicates electron capture (<sup>236</sup>Np), whereas white stands for a long-lasting [[nuclear isomer|metastable state]] (<sup>242</sup>Am).

The formation of actinide nuclides is primarily characterised by:<ref>Matthew W. Francis et al. (2014). [https://info.ornl.gov/sites/publications/Files/Pub52057.pdf Reactor fuel isotopics and code validation for nuclear applications]. ORNL/TM-2014/464, Oak Ridge, Tennessee, p. 11</ref>
* [[Neutron capture]] reactions (n,γ), which are represented in the figure by a short right arrow.
* The (n,2n) reactions and the less frequently occurring (γ,n) reactions are also taken into account, both of which are marked by a short left arrow.
* Even more rarely and only triggered by fast neutrons, the (n,3n) reaction occurs, which is represented in the figure with one example, marked by a long left arrow.

In addition to these neutron- or gamma-induced [[nuclear reaction]]s, the radioactive conversion of actinide nuclides also affects the nuclide inventory in a reactor. These decay types are marked in the figure by diagonal arrows. The [[beta decay|beta-minus decay]], marked with an arrow pointing up-left, plays a major role for the balance of the particle densities of the nuclides. Nuclides decaying by [[positron emission]] (beta-plus decay) or [[electron capture]] (ϵ) do not occur in a nuclear reactor except as products of knockout reactions; their decays are marked with arrows pointing down-right. Due to the long half-lives of the given nuclides, [[alpha decay]] plays almost no role in the formation and decay of the actinides in a power reactor, as the residence time of the nuclear fuel in the reactor core is rather short (a few years). Exceptions are the two relatively short-lived nuclides <sup>242</sup>Cm (T<sub>1/2</sub>&nbsp;= 163&nbsp;d) and <sup>236</sup>Pu (T<sub>1/2</sub>&nbsp;= 2.9&nbsp;y). Only for these two cases, the α decay is marked on the nuclide map by a long arrow pointing down-left. A few long-lived actinide isotopes, such as <sup>244</sup>Pu and <sup>250</sup>Cm, cannot be produced in reactors because neutron capture does not happen quickly enough to bypass the short-lived beta-decaying nuclides <sup>243</sup>Pu and <sup>249</sup>Cm; they can however be generated in nuclear explosions, which have much higher neutron fluxes.

== Distribution in nature ==
[[File:Uranium ore square.jpg|thumb|left|Unprocessed [[uranium ore]]]]

Thorium and uranium are the most abundant actinides in nature with the respective mass concentrations of 16&nbsp;ppm and 4&nbsp;ppm.<ref>{{cite book|url=https://books.google.com/books?id=w0wa4b9CGkcC&pg=SA2-PA38|pages=2–38|title=Standard handbook of environmental science, health, and technology|author1=Jay H. Lehr|author2=Janet K. Lehr|publisher=McGraw-Hill Professional|year=2000|isbn=978-0-07-038309-8}}</ref> Uranium mostly occurs in the Earth's crust as a mixture of its oxides in the mineral [[uraninite]], which is also called pitchblende because of its black color. There are several dozens of other [[:Category:Uranium minerals|uranium minerals]] such as [[carnotite]] (KUO<sub>2</sub>VO<sub>4</sub>·3H<sub>2</sub>O) and [[autunite]] (Ca(UO<sub>2</sub>)<sub>2</sub>(PO<sub>4</sub>)<sub>2</sub>·nH<sub>2</sub>O). The isotopic composition of natural uranium is [[Uranium-238|<sup>238</sup>U]] (relative abundance 99.2742%), [[Uranium-235|<sup>235</sup>U]] (0.7204%) and [[Uranium-234|<sup>234</sup>U]] (0.0054%); of these <sup>238</sup>U has the largest half-life of 4.51{{e|9}} years.<ref>{{RubberBible86th}}</ref><ref name="Yu. D. Tretyakov" /> The worldwide production of uranium in 2009 amounted to 50,572 [[tonne]]s, of which 27.3% was mined in [[Kazakhstan]]. Other important uranium mining countries are Canada (20.1%), Australia (15.7%), [[Namibia]] (9.1%), [[Russia]] (7.0%), and [[Niger]] (6.4%).<ref>{{cite web|url=http://www.world-nuclear.org/info/inf23.html|title=World Uranium Mining|publisher=World Nuclear Association|access-date=11 June 2010| archive-url=https://web.archive.org/web/20100626071100/http://www.world-nuclear.org/info/inf23.html|archive-date=26 June 2010|url-status=live}}</ref>

{| class="wikitable"  style="float:right; text-align:center;"
|+ Content of plutonium in uranium and thorium ores<ref name="katz" />
!Ore
!Location
! Uranium<br /> content, %
! Mass ratio <br /> <sup>239</sup>Pu/ore
! Ratio<br /> <sup>239</sup>Pu/U ({{e|-12}})
|-
| [[Uraninite]]|| Canada|| 13.5|| 9.1{{e|-12}}|| 7.1
|-
| Uraninite|| Congo|| 38|| 4.8{{e|-12}}|| 12
|-
| Uraninite|| [[Colorado]], US|| 50|| 3.8{{e|-12}}|| 7.7
|-
| [[Monazite]]|| Brazil|| 0.24|| 2.1{{e|-14}}|| 8.3
|-
| Monazite|| [[North Carolina]], US|| 1.64|| 5.9{{e|-14}}|| 3.6
|-
| [[Fergusonite]] ||-|| 0.25|| <1{{e|-14}}|| <4
|-
| [[Carnotite]] ||-|| 10|| <4{{e|-14}}|| <0.4
|}

The most abundant [[:Category:Thorium minerals|thorium minerals]] are [[thorianite]] ({{chem2|ThO2}}), [[thorite]] ({{chem2|ThSiO4}}) and [[monazite]], ({{chem2|(Th,Ca,Ce)PO4}}). Most thorium minerals contain uranium and vice versa; and they all have significant fraction of lanthanides. Rich deposits of thorium minerals are located in the United States (440,000 tonnes), Australia and India (~300,000 tonnes each) and Canada (~100,000 tonnes).<ref>[https://minerals.usgs.gov/minerals/pubs/commodity/thorium/mcs-2010-thori.pdf Thorium], USGS Mineral Commodities</ref>

The abundance of actinium in the Earth's crust is only about 5{{e|-15}}%.<ref name="Himiya protaktiniya" /> Actinium is mostly present in uranium-containing, but also in other minerals, though in much smaller quantities. The content of actinium in most natural objects corresponds to the isotopic equilibrium of parent isotope <sup>235</sup>U, and it is not affected by the weak Ac migration.<ref name="Himiya aktiniya" /> Protactinium is more abundant (10<sup>−12</sup>%) in the Earth's crust than actinium. It was discovered in uranium ore in 1913 by Fajans and Göhring.<ref name=fajans /> As actinium, the distribution of protactinium follows that of <sup>235</sup>U.<ref name="Himiya protaktiniya" />

The half-life of the longest-lived isotope of neptunium, [[Neptunium-237|<sup>237</sup>Np]], is negligible compared to the age of the Earth. Thus neptunium is present in nature in negligible amounts produced as intermediate decay products of other isotopes.<ref name="Himiya neptuniya" /> Traces of plutonium in uranium minerals were first found in 1942, and the more systematic results on <sup>239</sup>Pu are summarized in the table (no other plutonium isotopes could be detected in those samples). The upper limit of abundance of the longest-living isotope of plutonium, <sup>244</sup>Pu, is 3{{e|-20}}%. Plutonium could not be detected in samples of lunar soil. Owing to its scarcity in nature, most plutonium is produced synthetically.<ref name="katz" />

== Extraction ==
[[File:MonaziteUSGOV.jpg|thumb|upright=1.2|[[Monazite]]: a major thorium mineral]]

Owing to the low abundance of actinides, their extraction is a complex, multistep process. [[Fluoride]]s of actinides are usually used because they are insoluble in water and can be easily separated with [[redox]] reactions. Fluorides are reduced with [[calcium]], [[magnesium]] or [[barium]]:<ref name=g215>Golub, pp. 215–217</ref>
: <math chem>\begin{array}{l}{}\\
\ce{2AmF3{} + 3Ba ->[\ce{1150-1350^\circ C}] 3BaF2{} + 2Am}\\
\ce{PuF4{} + 2Ba ->[\ce{1200^\circ C}] 2BaF2{} + Pu}\\
\ce{UF4{} + 2Mg ->[\ce{> 500^\circ C}] U{} + 2MgF2}\\{}
\end{array}</math>

Among the actinides, thorium and uranium are the easiest to isolate. Thorium is extracted mostly from [[monazite]]: thorium [[pyrophosphate]] (ThP<sub>2</sub>O<sub>7</sub>) is reacted with [[nitric acid]], and the produced [[thorium nitrate]] treated with [[tributyl phosphate]]. [[Rare earth element|Rare-earth]] impurities are separated by increasing the [[pH]] in sulfate solution.<ref name=g215 />

In another extraction method, monazite is decomposed with a 45% aqueous solution of [[sodium hydroxide]] at 140&nbsp;°C. Mixed metal hydroxides are extracted first, filtered at 80&nbsp;°C, washed with water and dissolved with concentrated [[hydrochloric acid]]. Next, the acidic solution is neutralized with hydroxides to pH = 5.8 that results in precipitation of [[thorium hydroxide]] (Th(OH)<sub>4</sub>) contaminated with ~3% of rare-earth hydroxides; the rest of rare-earth hydroxides remains in solution. Thorium hydroxide is dissolved in an inorganic acid and then purified from the [[rare earth element]]s. An efficient method is the dissolution of thorium hydroxide in nitric acid, because the resulting solution can be purified by [[Liquid-liquid extraction|extraction]] with organic solvents:<ref name=g215 />

[[File:Plutonium and uranium extraction from nuclear fuel-eng.svg|thumb|right|upright=1.6|Separation of uranium and plutonium from [[spent nuclear fuel]] using the [[PUREX]] process<ref>Greenwood, pp. 1255, 1261</ref>]]

:Th(OH)<sub>4</sub> + 4 HNO<sub>3</sub> → Th(NO<sub>3</sub>)<sub>4</sub> + 4 H<sub>2</sub>O

Metallic thorium is separated from the anhydrous [[Thorium dioxide|oxide]], [[Thorium(IV) chloride|chloride]] or [[Thorium(IV) fluoride|fluoride]] by reacting it with calcium in an inert atmosphere:<ref name=g1255 />

:ThO<sub>2</sub> + 2 Ca → 2 CaO + Th

Sometimes thorium is extracted by [[electrolysis]] of a fluoride in a mixture of sodium and potassium chloride at 700–800&nbsp;°C in a [[graphite]] crucible. Highly pure thorium can be extracted from its iodide with the [[crystal bar process]].<ref>{{cite journal|doi=10.1002/zaac.19251480133|title=Darstellung von reinem Titanium-, Zirkonium-, Hafnium- und Thoriummetall|author=A. E. van Arkel|author2=de Boer, J. H.|volume=148|issue=1|pages=345–350|year=1925|journal=Zeitschrift für Anorganische und Allgemeine Chemie|language=de}}</ref>

Uranium is extracted from its ores in various ways. In one method, the ore is burned and then reacted with nitric acid to convert uranium into a dissolved state. Treating the solution with a solution of tributyl phosphate (TBP) in [[kerosene]] transforms uranium into an organic form UO<sub>2</sub>(NO<sub>3</sub>)<sub>2</sub>(TBP)<sub>2</sub>. The insoluble impurities are filtered and the uranium is extracted by reaction with hydroxides as [[Ammonium diuranate|(NH<sub>4</sub>)<sub>2</sub>U<sub>2</sub>O<sub>7</sub>]] or with [[hydrogen peroxide]] as [[Uranyl peroxide|UO<sub>4</sub>·2H<sub>2</sub>O]].<ref name=g215 />

When the uranium ore is rich in such minerals as [[dolomite (mineral)|dolomite]], [[magnesite]], etc., those minerals consume much acid. In this case, the carbonate method is used for uranium extraction. Its main component is an aqueous solution of [[sodium carbonate]], which converts uranium into a complex [UO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub>]<sup>4−</sup>, which is stable in aqueous solutions at low concentrations of hydroxide ions. The advantages of the sodium carbonate method are that the chemicals have low [[Corrosion|corrosivity]] (compared to nitrates) and that most non-uranium metals precipitate from the solution. The disadvantage is that tetravalent uranium compounds precipitate as well. Therefore, the uranium ore is treated with sodium carbonate at elevated temperature and under oxygen pressure:
:2 UO<sub>2</sub> + O<sub>2</sub> + 6 {{chem|CO|3|2-}} → 2 [UO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub>]<sup>4−</sup>
This equation suggests that the best solvent for the [[uranyl carbonate]] processing is a mixture of carbonate with bicarbonate. At high pH, this results in precipitation of [[diuranate]], which is treated with [[hydrogen]] in the presence of nickel yielding an insoluble uranium tetracarbonate.<ref name=g215 />

Another separation method uses polymeric resins as a [[polyelectrolyte]]. Ion exchange processes in the resins result in separation of uranium. Uranium from resins is washed with a solution of [[ammonium nitrate]] or nitric acid that yields [[uranyl nitrate]], UO<sub>2</sub>(NO<sub>3</sub>)<sub>2</sub>·6H<sub>2</sub>O. When heated, it turns into [[Uranium trioxide|UO<sub>3</sub>]], which is converted to [[uranium dioxide|UO<sub>2</sub>]] with hydrogen:
: UO<sub>3</sub> + H<sub>2</sub> → UO<sub>2</sub> + H<sub>2</sub>O
Reacting uranium dioxide with [[hydrofluoric acid]] changes it to [[uranium tetrafluoride]], which yields uranium metal upon reaction with magnesium metal:<ref name=g1255 />
: 4 HF + UO<sub>2</sub> → UF<sub>4</sub> + 2 H<sub>2</sub>O

To extract plutonium, neutron-irradiated uranium is dissolved in nitric acid, and a reducing agent ([[Iron(II) sulfate|FeSO<sub>4</sub>]], or [[hydrogen peroxide|H<sub>2</sub>O<sub>2</sub>]]) is added to the resulting solution. This addition changes the oxidation state of plutonium from +6 to +4, while uranium remains in the form of uranyl nitrate (UO<sub>2</sub>(NO<sub>3</sub>)<sub>2</sub>). The solution is treated with a reducing agent and neutralized with [[ammonium carbonate]] to pH = 8 that results in precipitation of Pu<sup>4+</sup> compounds.<ref name=g215 />

In another method, Pu<sup>4+</sup> and {{chem|UO|2|2+}} are first extracted with tributyl phosphate, then reacted with [[hydrazine]] washing out the recovered plutonium.<ref name=g215 />

The major difficulty in separation of actinium is the similarity of its properties with those of lanthanum. Thus actinium is either synthesized in nuclear reactions from isotopes of radium or separated using ion-exchange procedures.<ref name="Himiya aktiniya" />

== Properties ==
Actinides have similar properties to lanthanides. Just as the 4f electron shells are filled in the lanthanides, the 5f electron shells are filled in the actinides. Because the 5f, 6d, 7s, and 7p shells are close in energy, many irregular configurations arise; thus, in gas-phase atoms, just as the first 4f electron only appears in cerium, so the first 5f electron appears even later, in protactinium. However, just as lanthanum is the first element to use the 4f shell in compounds,<ref name="Hamilton">{{cite journal |last1=Hamilton |first1=David C. |date=1965 |title=Position of Lanthanum in the Periodic Table |journal=American Journal of Physics |volume=33 |issue=8 |pages=637–640 |doi=10.1119/1.1972042|bibcode=1965AmJPh..33..637H }}</ref> so actinium is the first element to use the 5f shell in compounds.<ref>{{cite journal |last1=Tomeček |first1=Josef |last2=Li |first2=Cen |first3=Georg |last3=Schreckenbach |date=2023 |title=Actinium coordination chemistry: A density functional theory study with monodentate and bidentate ligands |url= |journal=Journal of Computational Chemistry |volume=44 |issue=3 |pages=334–345 |doi=10.1002/jcc.26929 |pmid=35668552 |bibcode=2023JCoCh..44..334T |s2cid=249433367 |access-date=}}</ref> The f-shells complete their filling together, at ytterbium and nobelium.<ref name=johnson>{{cite book |last=Johnson |first=David |date=1984 |title=The Periodic Law |url=https://www.rsc.org/images/23_The_Periodic_Law_tcm18-30005.pdf |location= |publisher=The Royal Society of Chemistry |page= |isbn=0-85186-428-7}}</ref> The first experimental evidence for the filling of the 5f shell in actinides was obtained by McMillan and Abelson in 1940.<ref>{{cite book|author=I.L. Knunyants|title=Short Chemical Encyclopedia|place=Moscow|publisher=Soviet Encyclopedia|year=1961|volume=1}}</ref> As in lanthanides (see [[lanthanide contraction]]), the [[ionic radius]] of actinides monotonically decreases with atomic number (see also [[actinoid contraction]]).<ref>Golub, pp. 218–219</ref>

The shift of electron configurations in the gas phase does not always match the chemical behaviour. For example, the early-transition-metal-like prominence of the highest oxidation state, corresponding to removal of all valence electrons, extends up to uranium even though the 5f shells begin filling before that. On the other hand, electron configurations resembling the lanthanide congeners already begin at plutonium, even though lanthanide-like behaviour does not become dominant until the second half of the series begins at curium. The elements between uranium and curium form a transition between these two kinds of behaviour, where higher oxidation states continue to exist, but lose stability with respect to the +3 state.<ref name=johnson/> The +2 state becomes more important near the end of the series, and is the most stable oxidation state for nobelium, the last 5f element.<ref name=johnson/> Oxidation states rise again only after nobelium, showing that a new series of 6d transition metals has begun: [[lawrencium]] shows only the +3 oxidation state, and [[rutherfordium]] only the +4 state, making them respectively congeners of lutetium and hafnium in the 5d row.<ref name=johnson/>

{| Class = "wikitable collapsible" style="text-align:center;"
|+ Properties of actinides (the mass of the most long-lived isotope is in square brackets)<ref name="Yu. D. Tretyakov" /><ref name=g1263>Greenwood, p. 1263</ref>
!Element
! [[Actinium|Ac]]|| [[Thorium|Th]]|| [[Protactinium|Pa]]|| [[Uranium|U]]|| [[Neptunium|Np]]|| [[Plutonium|Pu]]|| [[Americium|Am]]|| [[Curium|Cm]]|| [[Berkelium|Bk]]|| [[Californium|Cf]]|| [[Einsteinium|Es]]|| [[Fermium|Fm]]|| [[Mendelevium|Md]]|| [[Nobelium|No]]|| [[Lawrencium|Lr]]
|-
!Core charge {{nobold|1=(''Z'')}}
! 89|| 90|| 91|| 92|| 93|| 94|| 95|| 96|| 97|| 98|| 99|| 100|| 101|| 102|| 103
|-
!Image
! [[File:Actinium_sample_(31481701837).png|50px]]||[[File:Thorium_sample_0.1g.jpg|50px]]||[[File:Protactinium_(Element_-_91)_2.jpg|50px]]||[[File:HEUraniumC.jpg|50px]]||[[File:Neptunium_(Element_-_93)_2.jpg|50px]]||[[File:Plutonium_ring.jpg|50px]]||[[File:Americium_microscope.jpg|50px]]||||[[File:Berkelium_metal.jpg|50px]]||[[File:Californium.jpg|50px]]||[[File:Einsteinium.jpg|50px]]||[[File:Fermium-Ytterbium_Alloy.jpg|50px]]||[[File:Tango-style question icon.svg|50px]]||[[File:Tango-style question icon.svg|50px]]||[[File:Tango-style question icon.svg|50px]]
|-
!Atomic mass
| [227]|| 232.0377(4) || 231.03588(2) || 238.02891(3) || [237]|| [244]|| [243]|| [247]|| [247]|| [251]|| [252]|| [257]|| [258]|| [259]|| [266]
|-
!Number of natural isotopes<ref name="emsley">{{cite book|author=John Emsley|title=Nature's Building Blocks: An A-Z Guide to the Elements|edition=New|year=2011|publisher=Oxford University Press|location=New York, NY|isbn=978-0-19-960563-7}}</ref>
| 3|| 8 || 3|| 8 || 3|| 4|| 0|| 0|| 0|| 0|| 0|| 0|| 0|| 0|| 0
|-
!Natural isotopes<ref name="emsley" /><ref>{{cite magazine|author=Peterson, Ivars|title=Uranium displays rare type of radioactivity|magazine=Science News|date=7 December 1991|url=http://findarticles.com/p/articles/mi_m1200/is_n23_v140/ai_11701241/|url-status=dead|archive-url=https://web.archive.org/web/20120118160007/http://findarticles.com/p/articles/mi_m1200/is_n23_v140/ai_11701241/|archive-date=18 January 2012}}</ref>
| 225, 227, 228 || 227–234 || 231, 233, 234 || 233–240 || 237, 239, 240 || 238–240, 244 || — || — || — || — || — || — || — || — || —
|-
!Natural quantity isotopes
| — || 230, 232 || 231 || 234, 235, 238 || — || — || — || — || — || — || — || — || — || — || —
|-
!Longest-lived isotope
| 227|| 232|| 231|| 238|| 237|| 244|| 243|| 247|| 247|| 251|| 252|| 257|| 258|| 259|| 266
|-
![[Half-life]] of the longest-lived isotope 
| {{val|21.8|u=years}} || {{convert|14|byr|yr|abbr=off}} || {{val|32500|u=years|fmt=commas}} || {{convert|4.47|byr|yr|abbr=off}} || {{convert|2.14|myr|yr|abbr=off}} || {{convert|80.8|myr|yr|abbr=off}} || {{val|7370|u=years|fmt=commas}} || {{convert|15.6|myr|yr|abbr=off}} || {{val|1380 |u=years|fmt=commas}} || {{val|900|u=years}} || {{val|1.29|u=years}} || {{val|100.5|ul=days}} || {{val|52|u=days}} || {{val|58|ul=min}} || {{val|11|ul=hours}}
|-
!Most common isotope
| 227 || 232 || 231 || 238 || 237 || 239 || 241 || 244 || 249 || 252 || 253 || 255 || 256 || 255 || 260
|-
!Half-life of the most common isotope
|{{val|21.8|u=years}}||{{convert|14|byr|yr|abbr=off}}||{{val|32500|u=years|fmt=commas}}||{{convert|4.47|byr|yr|abbr=off}}||{{convert|2.14|myr|yr|abbr=off}}||{{val|24100|u=years|fmt=commas}}||{{val|433|u=years}}||{{val|18.1|u=years}}||{{val|320|u=days}}||{{val|2.64|u=years}}||{{val|20.47|u=days}}||{{val|20.07|u=hours}}||{{val|78|u=min}}||{{val|3.1|u=min}}||{{val|2.7|u=min}}
|-
!Electronic configuration in<br />the ground state (gas phase)
| 6d<sup>1</sup>7s<sup>2</sup>|| 6d<sup>2</sup>7s<sup>2</sup>|| 5f<sup>2</sup>6d<sup>1</sup>7s<sup>2</sup>|| 5f<sup>3</sup>6d<sup>1</sup>7s<sup>2</sup>|| 5f<sup>4</sup>6d<sup>1</sup>7s<sup>2</sup> || 5f<sup>6</sup>7s<sup>2</sup>|| 5f<sup>7</sup>7s<sup>2</sup>|| 5f<sup>7</sup>6d<sup>1</sup>7s<sup>2</sup>|| 5f<sup>9</sup>7s<sup>2</sup> || 5f<sup>10</sup>7s<sup>2</sup>|| 5f<sup>11</sup>7s<sup>2</sup>|| 5f<sup>12</sup>7s<sup>2</sup>|| 5f<sup>13</sup>7s<sup>2</sup>|| 5f<sup>14</sup>7s<sup>2</sup>|| 5f<sup>14</sup>7s<sup>2</sup>7p<sup>1</sup>
|-
!Oxidation states
| 2, '''3'''|| 2, 3, '''4'''|| 2, 3, 4, '''5'''|| 2, 3, 4, 5, '''6'''|| 3, 4, '''5''', 6, 7|| 3, '''4''', 5, 6, 7|| 2, '''3''', 4, 5, 6, 7|| 2, '''3''', 4, 6|| 2, '''3''', 4|| 2, '''3''', 4|| 2, '''3''', 4|| 2, '''3'''|| 2, '''3'''|| '''2''', 3|| '''3'''
|-
!Metallic radius (nm)
| 0.203|| 0.180|| 0.162|| 0.153|| 0.150|| 0.162|| 0.173|| 0.174|| 0.170|| 0.186|| 0.186|| ?&nbsp;0.198|| ?&nbsp;0.194|| ?&nbsp;0.197|| ?&nbsp;0.171
|-
!{{nowrap|Ionic radius (nm):}}<br /> An<sup>4+</sup><br /> An<sup>3+</sup>
|<br /> — <br /> 0.126||<br /> 0.114<br /> —||<br /> 0.104<br /> 0.118||<br /> 0.103<br /> 0.118||<br /> 0.101<br /> 0.116||<br /> 0.100<br /> 0.115||<br /> 0.099<br /> 0.114||<br /> 0.099<br /> 0.112||<br /> 0.097<br /> 0.110||<br /> 0.096<br /> 0.109||<br /> 0.085<br /> 0.098||<br /> 0.084<br /> 0.091||<br /> 0.084<br /> 0.090||<br /> 0.084<br /> 0.095||<br /> 0.083<br /> 0.088
|- style="vertical-align:bottom"
!Temperature (°C):<br />melting<br /> boiling
| 1050<br />3198 || 1842<br />4788 || 1568<br />? 4027 || 1132.2<br />4131 || 639<br />? 4174 || 639.4<br />3228 || 1176<br />? 2607 || 1340<br />3110 || 986<br />2627 || <br />900<br />? 1470 || 860<br />? 996 || 1530<br />— || 830<br />— || 830<br />— || 1630<br />—
|-
!Density, g/cm<sup>3</sup>
|10.07 ||11.78 || 15.37|| 19.06||20.45||19.84||11.7||13.51||14.78||15.1||8.84||? 9.7||? 10.3||? 9.9||? 14.4
|- style="vertical-align:bottom"
!Standard electrode potential (V):<br />''E''° (An<sup>4+</sup>/An<sup>0</sup>)<br />''E''° (An<sup>3+</sup>/An<sup>0</sup>)
| —<br /> −2.13||<br /> −1.83<br /> —||<br /> −1.47<br /> —||<br /> −1.38<br /> −1.66||<br /> −1.30<br /> −1.79||<br /> −1.25<br /> −2.00||<br /> −0.90<br /> −2.07||<br /> −0.75<br /> −2.06||<br /> −0.55<br /> −1.96||<br /> −0.59<br /> −1.97||<br /> −0.36<br /> −1.98||<br /> −0.29<br /> −1.96||<br /> — <br /> −1.74||<br /> —<br /> −1.20||<br /> —<br /> −2.10
|- style="font-size:95%;" |
! style="font-size:110%;" | Color:<br /> [M(H<sub>2</sub>O)<sub>n</sub>]<sup>4+</sup><br /> [M(H<sub>2</sub>O)<sub>n</sub>]<sup>3+</sup>
|<br /> —<br /> Colorless||<br /> Colorless<br /> Blue||<br /> Yellow<br /> Dark blue||<br /> Green<br /> Purple||<br />{{nowrap|1=Yellow-green}}<br /> Purple|| <br /> Brown<br /> Violet||<br /> Red<br /> Rose||<br /> Yellow<br /> Colorless||<br /> Beige<br />{{nowrap|1=Yellow-green}}|| <br /> Green<br /> Green||<br /> —<br /> Pink||<br /> —<br /> —||<br /> —<br /> —||<br /> —<br /> —||<br /> —<br /> —
|}

{| class="wikitable" style="text-align:center; background:white"
|+
! colspan=16 | Approximate colors of actinide ions in aqueous solution<br />Colors for the actinides 100–103 are unknown as sufficient quantities have not yet been synthesized. The colour of {{chem2|CmO2(2+)}} was likewise not recorded.
<ref name="g1265">Greenwood, p. 1265</ref><ref>{{Cite journal|last1=Domanov|first1=V. P.|last2=Lobanov|first2=Yu. V.|date=October 2011|title=Formation of volatile curium(VI) trioxide CmO3|url=http://link.springer.com/10.1134/S1066362211050018|journal=Radiochemistry|language=en|volume=53|issue=5|pages=453–456|doi=10.1134/S1066362211050018|s2cid=98052484 |issn=1066-3622|url-access=subscription}}</ref>
|-
! style="text-align:right" | Actinide (''Z'') || 89 || 90 || 91 || 92 || 93 || 94 || 95 || 96 || 97 || 98 || 99 || 100 || 101 || 102 || 103
|-
! style="text-align:left" | Oxidation state || || || || || || || || || || || || || || ||
|-
! +2
| || || || || || || || || || || || <span style="color:silver;">'''Fm<sup>2+</sup>'''</span> || <span style="color:silver;">'''Md<sup>2+</sup>'''</span> || <span style="color:silver;">'''No<sup>2+</sup>'''</span> ||
|-
! +3
| '''Ac<sup>3+</sup>'''
| style="background:#00f;"| <span style="color:white;">'''Th<sup>3+</sup>'''</span>
| style="background:#007;"| <span style="color:white;">'''Pa<sup>3+</sup>'''</span>
| style="background:#c0b;"| <span style="color:white;">'''U<sup>3+</sup>'''</span>
| style="background:#b0d;"| <span style="color:white;">'''Np<sup>3+</sup>'''</span>
| style="background:#50f;"| <span style="color:white;">'''Pu<sup>3+</sup>'''</span>
| style="background:#fa9;"| '''Am<sup>3+</sup>'''
| '''Cm<sup>3+</sup>'''
| style="background:#cf0;"| '''Bk<sup>3+</sup>'''
| style="background:#0c0;"| '''Cf<sup>3+</sup>'''
| style="background:#fdd;"| '''Es<sup>3+</sup>'''
| <span style="color:silver;">'''Fm<sup>3+</sup>'''</span>
| <span style="color:silver;">'''Md<sup>3+</sup>'''</span>
| <span style="color:silver;">'''No<sup>3+</sup>'''</span>
| <span style="color:silver;">'''Lr<sup>3+</sup>'''</span>
|-
! +4
| || '''Th<sup>4+</sup>'''
| style="background:#ffd;"| '''Pa<sup>4+</sup>'''
| style="background:#0f0;"| '''U<sup>4+</sup>'''
| style="background:#cf0;"| '''Np<sup>4+</sup>'''
| style="background:#d60;"| '''Pu<sup>4+</sup>'''
| style="background:#fd0;"| '''Am<sup>4+</sup>'''
| style="background:#ffd;"| '''Cm<sup>4+</sup>'''
| style="background:#ffa;"| '''Bk<sup>4+</sup>'''
| style="background:#0c0;"| '''Cf<sup>4+</sup>'''
|
|
|
|
|
|-
! +5
| ||
| '''{{chem|PaO|2|+}}'''
| style="background:#ff88ff;"| '''{{chem|UO|2|+}}'''
| style="background:#0c0;"| '''{{chem|NpO|2|+}}'''
| style="background:#d17;"| <span style="color:white;">'''{{chem|PuO|2|+}}'''</span>
| style="background:#ff0;"| '''{{chem|AmO|2|+}}'''
|| || || || || || || ||
|-
! +6
| || ||
| style="background:#ff0;"| '''{{chem|UO|2|2+}}'''
| style="background:#fbb;"| '''{{chem|NpO|2|2+}}'''
| style="background:#fa7;"| '''{{chem|PuO|2|2+}}'''
| style="background:#ff5;"| '''{{chem|AmO|2|2+}}'''
| <span style="color:silver;">'''{{chem|CmO|2|2+}}'''</span>
|| || || || || || ||
|-
! +7
| || || ||
| style="background:#4a4;"| '''{{chem|NpO|2|3+}}'''
| style="background:#44aa88;"| '''{{chem|PuO|2|3+}}'''
| style="background:#4a4;"| '''{{chem|AmO|5|3−}}'''
|| || || || || || || ||
|}

=== Physical properties ===
{|class="wikitable" style ="text-align: center"
|-
| [[File:ActinidesLattice.png|400px]]
| [[File:ACTIION.PNG|400px]]
|-
|Major crystal structures of some actinides vs. temperature
|[[Metallic bond|Metallic]] and [[ionic radius|ionic]] radii of actinides<ref name="g1263" />
|}

[[File:Radioisotope thermoelectric generator plutonium pellet.jpg|thumb|A pellet of <sup>238</sup>PuO<sub>2</sub> to be used in a [[radioisotope thermoelectric generator]] for either the [[cassini spacecraft|Cassini]] or [[galileo spacecraft|Galileo]] mission. The pellet produces 62 watts of heat and glows because of the heat generated by the radioactive decay (primarily α). Photo is taken after insulating the pellet under a [[graphite]] blanket for minutes and removing the blanket.]]
[[File:Californium.jpg|thumb|left|[[Californium]]]]

Actinides are typical metals. All of them are soft and have a silvery color (but tarnish in air),<ref name=g1264>Greenwood, p. 1264</ref> relatively high [[density]] and plasticity. Some of them can be cut with a knife. Their [[electrical resistivity]] varies between 15 and 150 μΩ·cm.<ref name="g1263" /> The hardness of thorium is similar to that of soft steel, so heated pure thorium can be rolled in sheets and pulled into wire. Thorium is nearly half as dense as uranium and plutonium, but is harder than either of them. All actinides are radioactive, [[paramagnetism|paramagnetic]], and, with the exception of actinium, have several crystalline phases: plutonium has seven, and uranium, neptunium and californium three. The [[crystal structure]]s of protactinium, uranium, neptunium and plutonium do not have clear analogs among the lanthanides and are more similar to those of the 3''d''-[[transition metal]]s.<ref name="Yu. D. Tretyakov" />

All actinides are [[Pyrophoricity|pyrophoric]], especially when finely divided, that is, they spontaneously ignite upon reaction with air at room temperature.<ref name=g1264 /><ref>{{cite book|quote=Many actinide metals, hydrides, carbides, alloys and other compounds may ignite at room temperature in a finely divided state.|url=https://chem.libretexts.org/Bookshelves/Inorganic_Chemistry/Modules_and_Websites_(Inorganic_Chemistry)/Descriptive_Chemistry/Elements_Organized_by_Block/4_f-Block_Elements/The_Actinides/1General_Properties_and_Reactions_of_The_Actinides|title= General Properties and Reactions of the Actinides|date=22 May 2015 |publisher= LibreTexts}}</ref> The [[melting point]] of actinides does not have a clear dependence on the number of ''f''-electrons. The unusually low melting point of neptunium and plutonium (~640&nbsp;°C) is explained by [[Hybridization (chemistry)|hybridization]] of 5''f'' and 6''d'' orbitals and the formation of directional bonds in these metals.<ref name="Yu. D. Tretyakov" />

{| class="wikitable collapsible collapsed"  style="margin-left: 1em; width:75%;"
|+ Comparison of [[ionic radius|ionic radii]] of lanthanides and actinides<ref>Myasoedov, pp. 30–31</ref>
|-
! [[Lanthanide]]s
! Ln<sup>3+</sup>, Å
! Actinides
! An<sup>3+</sup>, Å
! An<sup>4+</sup>, Å
|-
| [[Lanthanum]]|| 1.061|| [[Actinium]]|| 1.11||–
|-
| [[Cerium]]|| 1.034|| [[Thorium]]|| 1.08|| 0.99
|-
| [[Praseodymium]]|| 1.013|| [[Protactinium]]|| 1.05|| 0.93
|-
| [[Neodymium]]|| 0.995|| [[Uranium]]|| 1.03|| 0.93
|-
| [[Promethium]]|| 0.979|| [[Neptunium]]|| 1.01|| 0.92
|-
| [[Samarium]]|| 0.964|| [[Plutonium]]|| 1.00|| 0.90
|-
| [[Europium]]|| 0.950|| [[Americium]]|| 0.99|| 0.89
|-
| [[Gadolinium]]|| 0.938|| [[Curium]]|| 0.98|| 0.88
|-
| [[Terbium]]|| 0.923|| [[Berkelium]] ||–||–
|-
| [[Dysprosium]]|| 0.908|| [[Californium]] ||–||–
|-
| [[Holmium]]|| 0.894|| [[Einsteinium]] ||–||–
|-
| [[Erbium]]|| 0.881|| [[Fermium]] ||–||–
|-
| [[Thulium]]|| 0.869|| [[Mendelevium]] ||–||–
|-
| [[Ytterbium]]|| 0.858|| [[Nobelium]] ||–||–
|-
| [[Lutetium]]|| 0.848|| [[Lawrencium]] ||–||–
|}

=== Chemical properties ===
Like the lanthanides, all actinides are highly reactive with [[halogen]]s and [[chalcogen]]s; however, the actinides react more easily. Actinides, especially those with a small number of 5''f''-electrons, are prone to [[Hybridization (chemistry)|hybridization]]. This is explained by the similarity of the electron energies at the 5''f'', 7''s'' and 6''d'' shells. Most actinides exhibit a larger variety of valence states, and the most stable are +6 for uranium, +5 for protactinium and neptunium, +4 for thorium and plutonium and +3 for actinium and other actinides.<ref name=g222>Golub, pp. 222–227</ref>

Actinium is chemically similar to lanthanum, which is explained by their similar ionic radii and electronic structures. Like lanthanum, actinium almost always has an oxidation state of +3 in compounds, but it is less reactive and has more pronounced [[Base (chemistry)|basic]] properties. Among other trivalent actinides Ac<sup>3+</sup> is least acidic, i.e. has the weakest tendency to hydrolyze in aqueous solutions.<ref name="Himiya aktiniya" /><ref name="Yu. D. Tretyakov" />

Thorium is rather active chemically. Owing to lack of [[electron]]s on 6''d'' and 5''f'' orbitals, tetravalent thorium compounds are colorless. At pH < 3, solutions of thorium salts are dominated by the cations [Th(H<sub>2</sub>O)<sub>8</sub>]<sup>4+</sup>. The Th<sup>4+</sup> ion is relatively large, and depending on the [[coordination number]] can have a radius between 0.95 and 1.14 Å. As a result, thorium salts have a weak tendency to hydrolyse. The distinctive ability of thorium salts is their high solubility both in water and polar organic solvents.<ref name="Yu. D. Tretyakov" />

Protactinium exhibits two valence states; the +5 is stable, and the +4 state easily oxidizes to protactinium(V). Thus tetravalent protactinium in solutions is obtained by the action of strong reducing agents in a hydrogen atmosphere. Tetravalent protactinium is chemically similar to uranium(IV) and thorium(IV). [[Fluoride]]s, [[phosphate]]s, [[hypophosphate]]s, [[iodate]]s and [[phenylarsonate]]s of protactinium(IV) are insoluble in water and dilute acids. Protactinium forms soluble [[carbonate]]s. The hydrolytic properties of pentavalent protactinium are close to those of [[tantalum]](V) and [[niobium]](V). The complex chemical behavior of protactinium is a consequence of the start of the filling of the 5''f'' shell in this element.<ref name="Himiya protaktiniya" />

Uranium has a valence from 3 to 6, the last being most stable. In the hexavalent state, uranium is very similar to the [[group 6 element]]s. Many compounds of uranium(IV) and uranium(VI) are [[non-stoichiometric compound|non-stoichiometric]], i.e. have variable composition. For example, the actual chemical formula of uranium dioxide is UO<sub>2+x</sub>, where ''x'' varies between −0.4 and 0.32. Uranium(VI) compounds are weak [[Oxidizing agent|oxidants]]. Most of them contain the linear "[[uranyl]]" group, {{chem|UO|2|2+}}. Between 4 and 6 ligands can be accommodated in an equatorial plane perpendicular to the uranyl group. The uranyl group acts as a [[hard acid]] and forms stronger complexes with oxygen-donor ligands than with nitrogen-donor ligands. {{chem|NpO|2|2+}} and {{chem|PuO|2|2+}} are also the common form of Np and Pu in the +6 oxidation state. Uranium(IV) compounds exhibit reducing properties, e.g., they are easily oxidized by atmospheric oxygen. Uranium(III) is a very strong reducing agent. Owing to the presence of d-shell, uranium (as well as many other actinides) forms [[organometallic compound]]s, such as U<sup>III</sup>(C<sub>5</sub>H<sub>5</sub>)<sub>3</sub> and U<sup>IV</sup>(C<sub>5</sub>H<sub>5</sub>)<sub>4</sub>.<ref name="Yu. D. Tretyakov" /><ref name=g1278>Greenwood, p. 1278</ref>

Neptunium has valence states from 3 to 7, which can be simultaneously observed in solutions. The most stable state in solution is +5, but the valence +4 is preferred in solid neptunium compounds. Neptunium metal is very reactive. Ions of neptunium are prone to hydrolysis and formation of [[coordination compound]]s.<ref name="Himiya neptuniya" />

Plutonium also exhibits valence states between 3 and 7 inclusive, and thus is chemically similar to neptunium and uranium. It is highly reactive, and quickly forms an oxide film in air. Plutonium reacts with [[hydrogen]] even at temperatures as low as 25–50&nbsp;°C; it also easily forms [[halide]]s and [[intermetallic compound]]s. Hydrolysis reactions of plutonium ions of different oxidation states are quite diverse. Plutonium(V) can enter [[polymerization]] reactions.<ref name="Plutoniy" /><ref name="Himiya plutoniya">{{cite book|author=M. S. Milyukova|title=Analytical chemistry of plutonium|place=Moscow|publisher=Nauka|year=1965|isbn=978-0-250-39918-5|url=https://archive.org/details/analyticalchemis00inst}}</ref>

The largest chemical diversity among actinides is observed in americium, which can have valence between 2 and 6. Divalent americium is obtained only in dry compounds and non-aqueous solutions ([[acetonitrile]]). Oxidation states +3, +5 and +6 are typical for aqueous solutions, but also in the solid state. Tetravalent americium forms stable solid compounds ([[Americium dioxide|dioxide]], [[Americium(IV) fluoride|fluoride]] and [[Americium(IV) hydroxide|hydroxide]]) as well as complexes in aqueous solutions. It was reported that in alkaline solution americium can be oxidized to the heptavalent state, but these data proved erroneous. The most stable valence of americium is 3 in aqueous solution and 3 or 4 in solid compounds.<ref name=m25>Myasoedov, pp. 25–29</ref>

Valence 3 is dominant in all subsequent elements up to lawrencium (with the exception of nobelium). Curium can be tetravalent in solids ([[Curium(IV) fluoride|fluoride]], [[Curium(IV) oxide|dioxide]]). Berkelium, along with a valence of +3, also shows the valence of +4, more stable than that of curium; the valence 4 is observed in solid [[Berkelium tetrafluoride|fluoride]] and [[Berkelium(IV) oxide|dioxide]]. The stability of Bk<sup>4+</sup> in aqueous solution is close to that of [[Cerium|Ce]]<sup>4+</sup>.<ref>{{Cite journal|last1=Deblonde|first1=Gauthier J.-P.|last2=Sturzbecher-Hoehne|first2=Manuel|last3=Jong|first3=Wibe A. de|last4=Brabec|first4=Jiri|last5=Corie Y. Ralston|last6=Illy|first6=Marie-Claire|last7=An|first7=Dahlia D.|last8=Rupert|first8=Peter B.|last9=Strong|first9=Roland K.|date=September 2017|title=Chelation and stabilization of berkelium in oxidation state +IV|journal=Nature Chemistry|volume=9|issue=9|pages=843–849|doi=10.1038/nchem.2759|pmid=28837177|issn=1755-4349|url=http://www.escholarship.org/uc/item/9zn3q96n|bibcode=2017NatCh...9..843D|osti=1436161 }}</ref> Only valence 3 was observed for californium, einsteinium and fermium. The divalent state is proven for mendelevium and nobelium, and in nobelium it is more stable than the trivalent state. Lawrencium shows valence 3 both in solutions and solids.<ref name=m25 />

The redox potential <chem>\mathit E_\frac{M^4+}{AnO2^2+}</chem> increases from −0.32 V in uranium, through 0.34 V (Np) and 1.04 V (Pu) to 1.34 V in americium revealing the increasing reduction ability of the An<sup>4+</sup> ion from americium to uranium. All actinides form AnH<sub>3</sub> hydrides of black color with salt-like properties. Actinides also produce [[carbide]]s with the general formula of AnC or AnC<sub>2</sub> ([[Uranium carbide|U<sub>2</sub>C<sub>3</sub>]] for uranium) as well as sulfides An<sub>2</sub>S<sub>3</sub> and AnS<sub>2</sub>.<ref name=g222 />

<gallery mode="packed" widths="160px" heights="120px">
File:Uranylnitrate_crystals.jpg|[[Uranyl nitrate]] (UO<sub>2</sub>(NO<sub>3</sub>)<sub>2</sub>)
File:U Oxstufen.jpg|Aqueous solutions of uranium III, IV, V, VI salts
File:Np ox st .jpg|Aqueous solutions of neptunium III, IV, V, VI, VII salts
File:Plutonium in solution.jpg|Aqueous solutions of plutonium III, IV, V, VI, VII salts
File:UCl4.jpg|[[Uranium tetrachloride]]
File:Uranium hexafluoride crystals sealed in an ampoule.jpg|[[Uranium hexafluoride]]
File:Yellowcake.jpg|[[Triuranium octoxide|U<sub>3</sub>O<sub>8</sub>]] ([[yellowcake]])
</gallery>

== Compounds ==

=== Oxides and hydroxides ===
{| Class = "wikitable collapsible collapsed" style="text-align:center;"
|+ Oxides of actinides<ref name="Himiya aktiniya" /><ref name="Himiya neptuniya" /><ref name="Himiya protaktiniya">{{cite book|author=E.S. Palshin|title=Analytical chemistry of protactinium|place=Moscow|publisher=Nauka|year=1968}}</ref><ref>Myasoedov, p. 88</ref><ref name="Tablitsa soedineniy" />
|-
! rowspan="2"|[[Chemical compounds|Compound]]
! rowspan="2"|Color
! rowspan="2"|Crystal symmetry, type
! colspan="3"|Lattice constants, Å
! rowspan="2"|Density, g/cm<sup>3</sup>
! rowspan="2"|Temperature, °C
|-
!''a''
!''b''
!''c''
|-
| Ac<sub>2</sub>O<sub>3</sub>|| White|| Hexagonal, La<sub>2</sub>O<sub>3</sub>|| 4.07 ||-|| 6.29|| 9.19|| –
|-
| PaO<sub>2</sub>||-|| Cubic, CaF<sub>2</sub>|| 5.505 ||-||-||-||-
|-
| Pa<sub>2</sub>O<sub>5</sub>|| White|| cubic, CaF<sub>2</sub><br /> Cubic<br /> Tetragonal<br /> Hexagonal<br /> Rhombohedral <br />Orthorhombic|| 5.446<br /> 10.891<br /> 5.429<br /> 3.817<br /> 5.425<br /> 6.92|| -<br /> -<br /> -<br /> -<br /> -<br /> 4.02|| -<br /> 10.992<br /> 5.503<br /> 13.22<br /> -<br /> 4. 18 ||-|| 700<br /> 700–1100<br /> 1000<br /> 1000–1200<br /> 1240–1400<br /> –
|-
| ThO<sub>2</sub>|| Colorless|| Cubic|| 5.59 ||-||-|| 9.87|| –
|-
| UO<sub>2</sub>|| Black-brown|| Cubic|| 5.47 ||-||-|| 10.9|| –
|-
| NpO<sub>2</sub>|| Greenish-brown|| Cubic, CaF<sub>2</sub>|| 5.424 ||-||-|| 11.1|| –
|-
| PuO|| Black|| Cubic, NaCl|| 4.96 ||-||-|| 13.9|| –
|-
| PuO<sub>2</sub>|| Olive green|| Cubic|| 5.39 ||-||-|| 11.44|| –
|-
| Am<sub>2</sub>O<sub>3</sub>|| Red-brown<br /> Red-brown|| Cubic, Mn<sub>2</sub>O<sub>3</sub><br /> Hexagonal, La<sub>2</sub>O<sub>3</sub>|| 11.03<br /> 3.817 ||-||-<br /> 5.971|| 10.57<br /> 11.7|| –
|-
| AmO<sub>2</sub>|| Black|| Cubic, CaF<sub>2</sub>|| 5.376 ||-||-||-||-
|-
| Cm<sub>2</sub>O<sub>3</sub>|| White<ref>According to other sources, cubic sesquioxide of curium is olive-green. See {{cite web|url = http://www.xumuk.ru/encyklopedia/2248.html|title=Соединения curium site XuMuK.ru|language = ru|access-date =11 July 2010| archive-url= https://web.archive.org/web/20100818211138/http://www.xumuk.ru/encyklopedia/2248.html|archive-date=18 August 2010|url-status=live}}</ref><br /> -<br /> -|| Cubic, Mn<sub>2</sub>O<sub>2</sub><br /> Hexagonal, LaCl<sub>3</sub><br /> Monoclinic, Sm<sub>2</sub>O<sub>3</sub>|| 11.01<br /> 3.80<br /> 14.28|| -<br /> -<br /> 3.65|| -<br /> 6<br /> 8.9|| 11.7|| –
|-
| CmO<sub>2</sub>|| Black|| Cubic, CaF<sub>2</sub>|| 5.37 ||-||-||-||-
|-
| Bk<sub>2</sub>O<sub>3</sub>|| Light brown|| Cubic, Mn<sub>2</sub>O<sub>3</sub>|| 10.886||-||-||-||-
|-
| BkO<sub>2</sub>|| Red-brown|| Cubic, CaF<sub>2</sub>|| 5.33 ||-||-||-||-
|-
| Cf<sub>2</sub>O<sub>3</sub><ref>The atmosphere during the synthesis affects the lattice parameters, which might be due to non-stoichiometry as a result of oxidation or reduction of the trivalent californium. Main form is the cubic oxide of californium(III).</ref>|| Colorless <br />Yellowish<br /> -|| Cubic, Mn<sub>2</sub>O<sub>3</sub><br /> Monoclinic, Sm<sub>2</sub>O<sub>3</sub><br /> Hexagonal, La<sub>2</sub>O<sub>3</sub>|| 10.79<br /> 14.12<br /> 3.72|| -<br /> 3.59<br /> -|| -<br /> 8.80<br /> 5.96 ||-||-
|-
| CfO<sub>2</sub>|| Black|| Cubic|| 5.31 ||-||-||-||-
|-
| Es<sub>2</sub>O<sub>3</sub>||-|| Cubic, Mn<sub>2</sub>O<sub>3</sub><br /> Monoclinic <br /> Hexagonal, La<sub>2</sub>O<sub>3</sub>|| 10.07<br /> 14.1<br /> 3.7|| -<br /> 3.59<br /> -|| -<br /> 8.80<br /> 6 ||-||-
|}

{| class="wikitable collapsible collapsed" style="text-align:center"
|+Approximate colors of actinide oxides<br />(most stable are bolded)<ref name=g1268>Greenwood, p. 1268</ref>
! Oxidation state
| 89 || 90 || 91 || 92 || 93 || 94 || 95 || 96 || 97 || 98 ||99
|-
| +3||'''Ac<sub>2</sub>O<sub>3</sub>'''|| || || ||
|bgcolor=black| <span style="color:white;">Pu<sub>2</sub>O<sub>3</sub></span>
| style="background:#fa7;"| Am<sub>2</sub>O<sub>3</sub>
| '''Cm<sub>2</sub>O<sub>3</sub>'''
| style="background:#dfe111;"| Bk<sub>2</sub>O<sub>3</sub>
| style="background:#cf0;"| '''Cf<sub>2</sub>O<sub>3</sub>'''
| '''Es<sub>2</sub>O<sub>3</sub>'''
|-
| +4|| || '''ThO<sub>2</sub>'''
|bgcolor=black| <span style="color:white;">PaO<sub>2</sub></span>
| style="background:#765538;"| <span style="color:white;">UO<sub>2</sub></span>
| style="background:#616639;"| '''NpO<sub>2</sub>'''
| style="background:#e1bb11;"| '''PuO<sub>2</sub>'''
|bgcolor=black| <span style="color:white;">'''AmO<sub>2</sub>'''</span>
|bgcolor=black| <span style="color:white;">CmO<sub>2</sub></span>
|bgcolor=brown| '''BkO<sub>2</sub>'''
|bgcolor=black| <span style="color:white;">CfO<sub>2</sub></span>
|
|-
| +5|| ||
| '''Pa<sub>2</sub>O<sub>5</sub>'''
|bgcolor=black| <span style="color:white;">U<sub>2</sub>O<sub>5</sub></span>
| style="background:#765538;"| <span style="color:white;">Np<sub>2</sub>O<sub>5</sub></span>
|| || || || || ||
|-
| +5,+6 || || ||
| style="background:#0d5e35;"| <span style="color:white;">'''U<sub>3</sub>O<sub>8</sub>'''</span>
|| || || || || || ||
|-
|| +6|| || ||
| style="background:#f89d1a;"| UO<sub>3</sub>
|| || || || || || ||
|}

{| Class = "wikitable collapsible" style="text-align: center"
|+ Dioxides of some actinides
|-
|  style="background:lightblue; text-align:left;"|[[Chemical formula]]
| [[Thorium dioxide|ThO<sub>2</sub>]]|| [[Protactinium(IV) oxide|PaO<sub>2</sub>]]|| [[Uranium dioxide|UO<sub>2</sub>]]|| [[Neptunium(IV) oxide|NpO<sub>2</sub>]]|| [[Plutonium(IV) oxide|PuO<sub>2</sub>]]|| [[Americium dioxide|AmO<sub>2</sub>]]|| CmO<sub>2</sub>|| BkO<sub>2</sub>|| CfO<sub>2</sub>
|-
|  style="background:lightblue; text-align:left;"|[[CAS Number]]
| 1314-20-1|| 12036-03-2|| 1344-57-6|| 12035-79-9|| 12059-95-9|| 12005-67-3|| 12016-67-0|| 12010-84-3|| 12015–10–0
|-
|  style="background:lightblue; text-align:left;"|[[Molar mass]]
| 264.04 || 263.035 || 270.03 || 269.047 || 276.063 || 275.06 || 270–284**|| 279.069 || 283.078
|-
|  style="background:lightblue; text-align:left;"|[[Melting point]]<ref>{{cite book|author1=L.R. Morss |author2=Norman M. Edelstein|author3=Jean Fuger|title=The Chemistry of the Actinide and Transactinide Elements (Set Vol.1–6)|url=https://books.google.com/books?id=9vPuV3A0UGUC&pg=PA2139|year=2011|publisher=Springer|isbn=978-94-007-0210-3|page=2139}}</ref>
| 3390&nbsp;°C||||2865&nbsp;°C||2547&nbsp;°C||2400&nbsp;°C||2175&nbsp;°C|| || ||
|-
|  style="background:lightblue; text-align:left;"|Crystal structure
| Colspan = "9"|[[File:CaF2 polyhedra.png|250px]]<br />'''An'''<sup>4+</sup>: <span style="color:silver; background:silver;">__</span>&nbsp;&nbsp;/&nbsp;&nbsp;O<sup>2−</sup>: <span style="color:#9c0; background:#9c0;">__</span>
|-
|  style="background:lightblue; text-align:left;"|[[Space group]]
| Colspan = "9"|Fm{{overline|3}}m
|-
|  style="background:lightblue; text-align:left;"|[[Coordination number]]
| Colspan = "9" |'''An'''[8], O[4]
|}
: <small>'''An''' – actinide <br />**Depending on the isotopes</small>

Some actinides can exist in several oxide forms such as An<sub>2</sub>O<sub>3</sub>, AnO<sub>2</sub>, An<sub>2</sub>O<sub>5</sub> and AnO<sub>3</sub>. For all actinides, oxides AnO<sub>3</sub> are [[Amphoterism|amphoteric]] and An<sub>2</sub>O<sub>3</sub>, AnO<sub>2</sub> and An<sub>2</sub>O<sub>5</sub> are basic, they easily react with water, forming bases:<ref name=g222 />

: An<sub>2</sub>O<sub>3</sub> + 3 H<sub>2</sub>O → 2 An(OH)<sub>3</sub>.
These bases are poorly soluble in water and by their activity are close to the [[hydroxide]]s of rare-earth metals.<ref name=g222 />
Np(OH)<sub>3</sub> has not yet been synthesized, [[Plutonium(III) hydroxide|Pu(OH)<sub>3</sub>]] has a blue color while [[Americium(III) hydroxide|Am(OH)<sub>3</sub>]] is pink and [[Curium(III) hydroxide|Cm(OH)<sub>3</sub>]] is colorless.<ref name="Tananaev">{{cite book|last1=Krivovichev|first1=Sergei|last2=Burns|first2=Peter|last3=Tananaev|first3=Ivan|title=Structural Chemistry of Inorganic Actinide Compounds|publisher=Elsevier|chapter=Chapter 3|isbn=978-0-08-046791-7|year=2006|pages=67–78|chapter-url=https://books.google.com/books?id=mV-phntexBQC&pg=PA67}}</ref> Bk(OH)<sub>3</sub> and Cf(OH)<sub>3</sub> are also known, as are tetravalent hydroxides for Np, Pu and Am and pentavalent for Np and Am.<ref name="Tananaev" />

The strongest base is of actinium. All compounds of actinium are colorless, except for black [[actinium sulfide]] (Ac<sub>2</sub>S<sub>3</sub>).<ref name="g222" /> Dioxides of tetravalent actinides crystallize in the [[cubic system]], same as in [[calcium fluoride]].

Thorium reacting with oxygen exclusively forms the dioxide:
: <chem>Th{} + O2 ->[\ce{1000^\circ C}] \overbrace{ThO2}^{Thorium~dioxide}</chem>

Thorium dioxide is a refractory material with the highest melting point among any known oxide (3390&nbsp;°C).<ref name="g1268" /> Adding 0.8–1% ThO<sub>2</sub> to tungsten stabilizes its structure, so the doped filaments have better mechanical stability to vibrations. To dissolve ThO<sub>2</sub> in acids, it is heated to 500–600&nbsp;°C; heating above 600&nbsp;°C produces a very resistant to acids and other reagents form of ThO<sub>2</sub>. Small addition of fluoride ions [[Catalyst|catalyses]] dissolution of thorium dioxide in acids.

Two protactinium oxides have been obtained: PaO<sub>2</sub> (black) and Pa<sub>2</sub>O<sub>5</sub> (white); the former is isomorphic with ThO<sub>2</sub> and the latter is easier to obtain. Both oxides are basic, and Pa(OH)<sub>5</sub> is a weak, poorly soluble base.<ref name=g222 />

Decomposition of certain salts of uranium, for example UO<sub>2</sub>(NO<sub>3</sub>)·6H<sub>2</sub>O in air at 400&nbsp;°C, yields orange or yellow UO<sub>3</sub>.<ref name="g1268" /> This oxide is amphoteric and forms several hydroxides, the most stable being [[uranyl hydroxide]] UO<sub>2</sub>(OH)<sub>2</sub>. Reaction of uranium(VI) oxide with hydrogen results in uranium dioxide, which is similar in its properties with ThO<sub>2</sub>. This oxide is also basic and corresponds to the uranium hydroxide U(OH)<sub>4</sub>.<ref name=g222 />

Plutonium, neptunium and americium form two basic oxides: An<sub>2</sub>O<sub>3</sub> and AnO<sub>2</sub>. Neptunium trioxide is unstable; thus, only Np<sub>3</sub>O<sub>8</sub> could be obtained so far. However, the oxides of plutonium and neptunium with the chemical formula AnO<sub>2</sub> and An<sub>2</sub>O<sub>3</sub> are well characterized.<ref name=g222 />

=== Salts ===

{| Class = "wikitable collapsible" style="text-align: center"
|+ Trichlorides of some actinides<ref name=g1270>Greenwood, p. 1270</ref>
|-
! style="background:lightblue; text-align:left;"|[[Chemical formula]]
| AcCl<sub>3</sub>|| UCl<sub>3</sub>|| NpCl<sub>3</sub>|| PuCl<sub>3</sub>|| AmCl<sub>3</sub>|| CmCl<sub>3</sub>|| BkCl<sub>3</sub>|| CfCl<sub>3</sub>
|-
! style="background:lightblue; text-align:left;"|[[CAS-number]]
| 22986-54-5|| 10025-93-1|| 20737-06-8|| 13569-62-5|| 13464-46-5|| 13537-20-7|| 13536-46-4|| 13536–90–8
|-
! style="background:lightblue; text-align:left;"|[[Molar mass]]
| 333.386 || 344.387 || 343.406 || 350.32 || 349.42 || 344–358**|| 353.428 || 357.438
|-
! style="background:lightblue; text-align:left;"|[[Melting point]]
||| 837&nbsp;°C||800&nbsp;°C||767&nbsp;°C||715&nbsp;°C||695&nbsp;°C||603&nbsp;°C||545&nbsp;°C
|-
! style="background:lightblue; text-align:left;"|[[Boiling point]]
||| 1657&nbsp;°C||||1767&nbsp;°C||850&nbsp;°C|| || ||
|-
! style="background:lightblue; text-align:left;"|Crystal structure
| Colspan = "8"|[[File:UCl3 without caption.png|250px|The crystal structure of uranium trichloride]]<br />'''An'''<sup>3+</sup>: <span style="color:silver; background:silver;">__</span>&nbsp;&nbsp;/&nbsp;&nbsp;Cl<sup>−</sup>: <span style="color:#0f0; background:#0f0;">__</span>
|-
! style="background:lightblue; text-align:left;"|[[Space group]]
| Colspan = "8"|P6<sub>3</sub>/m
|-
! style="background:lightblue; text-align:left;"|[[Coordination number]]
| Colspan = "8" |'''An'''*[9], Cl [3]
|-
! style="background:lightblue; text-align:left;"|Lattice constants
| ''a'' = 762 [[picometres|pm]]<br /> ''c'' = 455 pm|| ''a'' = 745.2 pm<br /> ''c'' = 432.8 pm|| || ''a'' = 739.4 pm<br /> ''c'' = 424.3 pm|| ''a'' = 738.2 pm<br /> ''c'' = 421.4 pm|| ''a'' = 726 pm<br /> ''c'' = 414 pm|| ''a'' = 738.2 pm<br /> ''c'' = 412.7 pm|| ''a'' = 738 pm <br /> ''c'' = 409 pm
|}
:<small> *'''An''' – actinide <br />**Depending on the isotopes</small>

{| Class = "wikitable collapsible collapsed" style="text-align: center"
|+ Actinide fluorides<ref name="Himiya neptuniya" /><ref name="Himiya protaktiniya" /><ref name="Tablitsa soedineniy">{{cite web|url=http://chemanalytica.com/book/novyy_spravochnik_khimika_i_tekhnologa/01_osnovnye_svoystva_neorganicheskikh_organicheskikh_i_elementoorganicheskikh_soedineniy|title=Таблица Inorganic and Coordination compounds|language = ru|access-date =11 July 2010}}</ref><ref name=g1270 /><ref>Myasoedov, pp. 96–99</ref>
|-
! Rowspan = "2"|Compound
! rowspan="2"|Color
! rowspan="2"|Crystal symmetry, type
! colspan="3"|Lattice constants, Å
! rowspan="2"|Density, g/cm<sup>3</sup>
|-
!''a''
!''b''
!''c''
|-
| AcF<sub>3</sub>|| White|| Hexagonal, LaF<sub>3</sub>|| 4.27 ||-|| 7.53|| 7.88
|-
| PaF<sub>4</sub>|| Dark brown|| [[Monoclinic]]|| 12.7|| 10.7|| 8.42|| –
|-
| PaF<sub>5</sub>|| Black|| [[Tetragonal]], β-UF<sub>5</sub>|| 11.53 ||-|| 5.19|| –
|-
| ThF<sub>4</sub>|| Colorless|| Monoclinic|| 13|| 10.99|| 8.58|| 5.71
|-
| UF<sub>3</sub>|| Reddish-purple|| Hexagonal|| 7.18 ||-|| 7.34|| 8.54
|-
| UF<sub>4</sub>|| Green|| Monoclinic|| 11.27|| 10.75|| 8.40|| 6.72
|-
| α-UF<sub>5</sub>|| Bluish|| Tetragonal|| 6.52 ||-|| 4.47|| 5.81
|-
| β-UF<sub>5</sub>|| Bluish|| Tetragonal|| 11.47 ||-|| 5.20|| 6.45
|-
| UF<sub>6</sub>|| Yellowish|| Orthorhombic|| 9.92|| 8.95|| 5.19|| 5.06
|-
| NpF<sub>3</sub>|| Black or purple|| Hexagonal|| 7.129 ||-|| 7.288|| 9.12
|-
| NpF<sub>4</sub>|| Light green|| Monoclinic|| 12.67|| 10.62|| 8.41|| 6.8
|-
| NpF<sub>6</sub>|| Orange|| Orthorhombic|| 9.91|| 8.97|| 5.21|| 5
|-
| PuF<sub>3</sub>|| Violet-blue|| Trigonal|| 7.09 ||-|| 7.25|| 9.32
|-
| PuF<sub>4</sub>|| Pale brown|| Monoclinic|| 12.59|| 10.57|| 8.28|| 6.96
|-
| PuF<sub>6</sub>|| Red-brown|| Orthorhombic|| 9.95|| 9.02|| 3.26|| 4.86
|-
| AmF<sub>3</sub>|| Pink or light beige|| [[Hexagonal crystal system|hexagonal]], LaF<sub>3</sub>|| 7.04<ref name="katz">{{cite book|author = F. Weigel|title = The Chemistry of the Actinide Elements|place=Moscow|publisher = Mir| year = 1997|volume = 2|isbn = 978-5-03-001885-0|author2 = J. Katz|author3 = G. Seaborg}}</ref><ref>{{cite journal|last1=Nave|first1=S.|last2=Haire|first2=R.|last3=Huray|first3=Paul|title=Magnetic properties of actinide elements having the 5f<sup>6</sup> and 5f<sup>7</sup> electronic configurations|journal=Physical Review B|volume=28|issue=5|pages=2317–2327|year=1983|doi=10.1103/PhysRevB.28.2317|bibcode = 1983PhRvB..28.2317N }}</ref>||-|| 7.255|| 9.53
|-
| AmF<sub>4</sub>|| Orange-red|| [[Monoclinic]]|| 12.53|| 10.51|| 8.20|| –
|-
| CmF<sub>3</sub>|| From brown to white|| Hexagonal|| 4.041 ||-|| 7.179|| 9.7
|-
| CmF<sub>4</sub>|| Yellow|| Monoclinic, UF<sub>4</sub>|| 12.51|| 10.51|| 8.20|| –
|-
| BkF<sub>3</sub>|| Yellow-green|| [[Trigonal]], LaF<sub>3</sub><br /> [[Orthorhombic]], YF<sub>3</sub>|| 6.97<br /> 6.7|| -<br /> 7.09|| 7.14<br /> 4.41|| 10.15<br /> 9.7
|-
| BkF<sub>4</sub>||-|| Monoclinic, UF<sub>4</sub>|| 12.47|| 10.58|| 8.17|| –
|-
| CfF<sub>3</sub>|| -<br /> -|| Trigonal, LaF<sub>3</sub><br /> Orthorhombic, YF<sub>3</sub>|| 6. 94<br /> 6.65|| -<br /> 7.04|| 7.10<br /> 4.39|| –
|-
| CfF<sub>4</sub>|| -<br /> -|| Monoclinic, UF<sub>4</sub><br /> Monoclinic, UF<sub>4</sub>|| 1.242 <br /> 1.233|| 1.047<br /> 1.040|| 8.126<br /> 8.113|| –
|}

[[File:Einsteinium triiodide by transmitted light.jpg|thumb|left|[[Einsteinium triiodide]] glowing in the dark]]
Actinides easily react with halogens forming salts with the formulas MX<sub>3</sub> and MX<sub>4</sub> (X = [[halogen]]). So the first berkelium compound, [[Berkelium(III) chloride|BkCl<sub>3</sub>]], was synthesized in 1962 with an amount of 3 nanograms. Like the halogens of rare earth elements, actinide [[chloride]]s, [[bromide]]s, and [[iodide]]s are water-soluble, and [[fluoride]]s are insoluble. Uranium easily yields a colorless hexafluoride, which [[Sublimation (phase transition)|sublimates]] at a temperature of 56.5&nbsp;°C; because of its volatility, it is used in the separation of uranium isotopes with [[gas centrifuge]] or [[gaseous diffusion]]. Actinide hexafluorides have properties close to [[anhydride]]s. They are very sensitive to moisture and hydrolyze forming AnO<sub>2</sub>F<sub>2</sub>.<ref name=g1269>Greenwood, p.1269</ref> The [[Uranium pentachloride|pentachloride]] and black [[Uranium hexachloride|hexachloride]] of uranium were synthesized, but they are both unstable.<ref name=g222 />

Action of acids on actinides yields salts, and if the acids are non-oxidizing then the actinide in the salt is in low-valence state:
: U + 2 [[Sulfuric acid|H<sub>2</sub>SO<sub>4</sub>]] → [[Uranium(IV) sulfate|U(SO<sub>4</sub>)<sub>2</sub>]] + 2 H<sub>2</sub>
: 2 Pu + 6 [[Hydrochloric acid|HCl]] → 2 [[Plutonium(III) chloride|PuCl<sub>3</sub>]] + 3 H<sub>2</sub>
However, in these reactions the regenerating hydrogen can react with the metal, forming the corresponding hydride. Uranium reacts with acids and water much more easily than thorium.<ref name=g222 />

Actinide salts can also be obtained by dissolving the corresponding hydroxides in acids. Nitrates, chlorides, sulfates and perchlorates of actinides are water-soluble. When crystallizing from aqueous solutions, these salts form hydrates, such as [[Thorium(IV) nitrate|Th(NO<sub>3</sub>)<sub>4</sub>·6H<sub>2</sub>O]], [[Thorium(IV) sulfate|Th(SO<sub>4</sub>)<sub>2</sub>·9H<sub>2</sub>O]] and [[Plutonium(III) sulfate|Pu<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub>·7H<sub>2</sub>O]]. Salts of high-valence actinides easily hydrolyze. So, colorless sulfate, chloride, perchlorate and nitrate of thorium transform into basic salts with formulas Th(OH)<sub>2</sub>SO<sub>4</sub> and Th(OH)<sub>3</sub>NO<sub>3</sub>. The solubility and insolubility of trivalent and tetravalent actinides is like that of lanthanide salts. So [[phosphate]]s, [[fluoride]]s, [[oxalate]]s, [[iodate]]s and [[carbonate]]s of actinides are weakly soluble in water; they precipitate as hydrates, such as [[Thorium(IV) fluoride|ThF<sub>4</sub>·3H<sub>2</sub>O]] and [[Thorium(IV) chromate|Th(CrO<sub>4</sub>)<sub>2</sub>·3H<sub>2</sub>O]].<ref name=g222 />

Actinides with oxidation state +6, except for the AnO<sub>2</sub><sup>2+</sup>-type cations, form [AnO<sub>4</sub>]<sup>2−</sup>, [An<sub>2</sub>O<sub>7</sub>]<sup>2−</sup> and other complex anions. For example, uranium, neptunium and plutonium form salts of the Na<sub>2</sub>UO<sub>4</sub> ([[uranate]]) and (NH<sub>4</sub>)<sub>2</sub>U<sub>2</sub>O<sub>7</sub> (diuranate) types. In comparison with lanthanides, actinides more easily form [[coordination compound]]s, and this ability increases with the actinide valence. Trivalent actinides do not form fluoride coordination compounds, whereas tetravalent thorium forms K<sub>2</sub>ThF<sub>6</sub>, KThF<sub>5</sub>, and even K<sub>5</sub>ThF<sub>9</sub> complexes. Thorium also forms the corresponding [[sulfate]]s (for example Na<sub>2</sub>SO<sub>4</sub>·Th(SO<sub>4</sub>)<sub>2</sub>·5H<sub>2</sub>O), [[nitrate]]s and [[thiocyanate]]s. Salts with the general formula An<sub>2</sub>Th(NO<sub>3</sub>)<sub>6</sub>·''n''H<sub>2</sub>O are of coordination nature, with the [[coordination number]] of thorium equal to 12. Even easier is to produce complex salts of pentavalent and hexavalent actinides. The most stable coordination compounds of actinides – tetravalent thorium and uranium – are obtained in reactions with diketones, e.g. [[acetylacetone]].<ref name=g222 />

== Applications ==
[[File:InsideSmokeDetector.jpg|thumb|Interior of a [[smoke detector]] containing [[americium-241]].]]
While actinides have some established daily-life applications, such as in smoke detectors (americium)<ref>[https://web.archive.org/web/19960101/http://www.uic.com.au/nip35.htm Smoke Detectors and Americium], Nuclear Issues Briefing Paper 35, May 2002</ref><ref name=g1262>Greenwood, p. 1262</ref> and [[gas mantle]]s (thorium),<ref name=g1255>Greenwood, p. 1255</ref> they are mostly used in [[nuclear weapon]]s and as [[nuclear fuel|fuel]] in nuclear reactors.<ref name=g1255 /> The last two areas exploit the property of actinides to release enormous energy in nuclear reactions, which under certain conditions may become self-sustaining [[Nuclear chain reaction|chain reactions]].

[[File:Cerenkov Effect.jpg|thumb|left|upright|Self-illumination of a nuclear reactor by [[Cherenkov radiation]].]]
The most important isotope for [[nuclear power]] applications is [[uranium-235]]. It is used in the [[thermal reactor]], and its concentration in natural uranium does not exceed 0.72%. This isotope strongly absorbs [[thermal neutron]]s releasing much energy. One fission act of 1&nbsp;gram of <sup>235</sup>U converts into about 1 MW·day. Of importance, is that {{nuclide|U|235}} emits more neutrons than it absorbs;<ref name=g220>Golub, pp. 220–221</ref> upon reaching the [[critical mass]], {{nuclide|U|235}} enters into a self-sustaining chain reaction.<ref name="Yu. D. Tretyakov">{{cite book|editor=Yu.D. Tretyakov|title=Non-organic chemistry in three volumes|place=Moscow|publisher=Academy|year=2007|volume=3|series=Chemistry of transition elements|isbn=978-5-7695-2533-9}}</ref> Typically, uranium nucleus is divided into two fragments with the release of 2–3 neutrons, for example:
: {{nuclide|U|235|link=yes}} + {{nuclide|neutronium|1|link=yes}} ⟶ {{nuclide|Rh|115}} + {{Nuclide|Ag|118}} + 3{{nuclide|neutronium|1}}

Other promising actinide isotopes for nuclear power are [[thorium-232]] and its product from the [[thorium fuel cycle]], [[uranium-233]].
{| class="wikitable"  style="float:right; width:40%;"
|-  style="background:lightblue; text-align:center;"
| [[Nuclear reactor]]<ref name="Yu. D. Tretyakov" /><ref>{{cite book|author1=G. G. Bartolomei|author2=V. D. Baybakov|author3=M. S. Alkhutov|author4=G. A. Bach|title=Basic theories and methods of calculation of nuclear reactors|location=Moscow|publisher=Energoatomizdat|year=1982}}</ref><ref>Greenwood, pp. 1256–1261</ref>
|-
| <small> The core of most [[Generation II reactor|Generation II nuclear reactors]] contains a set of hollow metal rods, usually made of [[zirconium]] alloys, filled with solid [[nuclear fuel]] pellets – mostly oxide, carbide, nitride or monosulfide of uranium, plutonium or thorium, or their mixture (the so-called [[MOX fuel]]). The most common fuel is oxide of uranium-235.</small>
[[File:Heterogeneous reactor scheme.png|border|150px|left|Nuclear reactor scheme]]
<small>[[Neutron temperature|Fast neutrons]] are slowed by [[Moderator (Nuclear Reactor)|moderators]], which contain water, [[carbon]], [[deuterium]], or [[beryllium]], as [[thermal neutrons]] to increase the efficiency of their interaction with uranium-235. The rate of nuclear reaction is controlled by introducing additional rods made of [[boron]] or [[cadmium]] or a liquid absorbent, usually [[boric acid]]. Reactors for plutonium production are called [[breeder reactor]] or breeders; they have a different design and use fast neutrons.</small>
|}
Emission of neutrons during the fission of uranium is important not only for maintaining the nuclear chain reaction, but also for the synthesis of the heavier actinides. [[Uranium-239]] converts via [[Beta decay|β-decay]] into plutonium-239, which, like uranium-235, is capable of spontaneous fission. The world's first nuclear reactors were built not for energy, but for producing plutonium-239 for nuclear weapons.

About half of produced thorium is used as the light-emitting material of gas mantles.<ref name=g1255 /> Thorium is also added into multicomponent [[alloy]]s of [[magnesium]] and [[zinc]]. Mg-Th alloys are light and strong, but also have high melting point and ductility and thus are widely used in the aviation industry and in the production of [[missile]]s. Thorium also has good [[electron emission]] properties, with long lifetime and low potential barrier for the emission.<ref name=g220 /> The relative content of thorium and uranium isotopes is widely used to estimate the age of various objects, including stars (see [[:Category:Radiometric dating|radiometric dating]]).<ref>{{cite journal|author1=Sergey Popov|author2=Alexander Sergeev|title=Universal Alchemy|url=http://www.vokrugsveta.ru/vs/article/6214/|language=ru|journal=Vokrug Sveta|year=2008|volume=2811|issue=4}}</ref>

The major application of plutonium has been in [[nuclear weapon]]s, where the isotope plutonium-239 was a key component due to its ease of fission and availability. Plutonium-based designs allow reducing the [[critical mass (nuclear)|critical mass]] to about a third of that for uranium-235.<ref>{{cite book|author=David L. Heiserman|title=Exploring Chemical Elements and their Compounds|location=New York|year=1992|publisher=TAB Books|isbn=978-0-8306-3018-9|chapter=Element 94: Plutonium|page=338|chapter-url=https://archive.org/details/exploringchemica01heis/page/338}}</ref> The "[[Fat Man]]"-type plutonium bombs produced during the [[Manhattan Project]] used explosive compression of plutonium to obtain significantly higher densities than normal, combined with a central neutron source to begin the reaction and increase efficiency. Thus only 6.2&nbsp;kg of plutonium was needed for an [[Nuclear weapon yield|explosive yield]] equivalent to 20 kilotons of [[Trinitrotoluene|TNT]].<ref>{{cite book|author=John Malik|url=https://fas.org/sgp/othergov/doe/lanl/docs1/00313791.pdf|title=The Yields of the Hiroshima and Nagasaki Explosions|publisher=Los Alamos|id=LA-8819|date=September 1985|page=Table VI|access-date=15 February 2009|archive-url=https://web.archive.org/web/20090224204106/https://fas.org/sgp/othergov/doe/lanl/docs1/00313791.pdf|archive-date=24 February 2009|url-status=live}}</ref> (See also [[Nuclear weapon design]].) Hypothetically, as little as 4&nbsp;kg of plutonium—and maybe even less—could be used to make a single atomic bomb using very sophisticated assembly designs.<ref>{{cite web|url=https://fas.org/nuke/intro/nuke/design.htm|title=Nuclear Weapon Design|publisher=Federation of American Scientists|year=1998|access-date=7 December 2008|archive-url=https://web.archive.org/web/20081226091803/https://fas.org/nuke/intro/nuke/design.htm|archive-date=26 December 2008|url-status=dead}}</ref>

[[Plutonium-238]] is potentially more efficient isotope for nuclear reactors, since it has smaller critical mass than uranium-235, but it continues to release much thermal energy (0.56 W/g)<ref name=g1262 /><ref>John Holdren and Matthew Bunn [https://web.archive.org/web/20101105035505/http://www.nti.org/e_research/cnwm/overview/technical2.asp Nuclear Weapons Design & Materials]. Project on Managing the Atom (MTA) for NTI. 25 November 2002</ref> by decay even when the fission chain reaction is stopped by control rods. Its application is limited by its high price (about US$1000/g). This isotope has been used in [[thermopile]]s and water [[distillation]] systems of some space satellites and stations. The [[Galileo (spacecraft)|Galileo]] and [[Apollo program|Apollo]] spacecraft (e.g. [[Apollo 14]]<ref>[http://www.hq.nasa.gov/alsj/a14/A14_PressKit.pdf Apollo 14 Press Kit – 01/11/71] {{Webarchive|url=https://web.archive.org/web/20190721143247/http://www.hq.nasa.gov/alsj/a14/A14_PressKit.pdf |date=21 July 2019 }}, NASA, pp. 38–39</ref>) had heaters powered by kilogram quantities of plutonium-238 oxide; this heat is also transformed into electricity with thermopiles. The decay of plutonium-238 produces relatively harmless alpha particles and is not accompanied by [[gamma ray]]s. Therefore, this isotope (~160&nbsp;mg) is used as the energy source in heart pacemakers where it lasts about 5 times longer than conventional batteries.<ref name=g1262 />

[[Actinium-227]] is used as a neutron source. Its high specific energy (14.5 W/g) and the possibility of obtaining significant quantities of thermally stable compounds are attractive for use in long-lasting thermoelectric generators for remote use. <sup>228</sup>Ac is used as an indicator of [[radioactivity]] in chemical research, as it emits high-energy electrons (2.18 MeV) that can be easily detected. [[Actinium-228|<sup>228</sup>Ac]]-[[Radium-228|<sup>228</sup>Ra]] mixtures are widely used as an intense gamma-source in industry and medicine.<ref name="Himiya aktiniya" />

Development of self-glowing actinide-doped materials with durable crystalline matrices is a new area of actinide utilization as the addition of alpha-emitting radionuclides to some glasses and crystals may confer luminescence.<ref name=burakov>{{cite book|author1=B.E. Burakov|author2=M.I Ojovan|author3=W.E. Lee|title=Crystalline Materials for Actinide Immobilisation|publisher=World Scientific|year=2010|url=https://books.google.com/books?id=BWriuXxa7CYC|isbn=978-1-84816-418-5}}</ref>

== Toxicity ==
[[File:Alfa beta gamma radiation penetration.svg|thumb|Schematic illustration of penetration of radiation through sheets of paper, aluminium and lead brick]]
{{Periodic table (transuranium element)}}

Radioactive substances can harm human health via (i) local skin contamination, (ii) internal exposure due to ingestion of radioactive isotopes, and (iii) external overexposure by [[Beta particle|β-activity]] and [[Gamma ray|γ-radiation]]. Together with radium and transuranium elements, actinium is one of the most dangerous radioactive poisons with high specific [[Alpha radiation|α-activity]]. The most important feature of actinium is its ability to accumulate and remain in the surface layer of [[skeleton]]s. At the initial stage of poisoning, actinium accumulates in the [[liver]]. Another danger of actinium is that it undergoes radioactive decay faster than being excreted. [[Adsorption]] from the digestive tract is much smaller (~0.05%) for actinium than radium.<ref name="Himiya aktiniya" />

Protactinium in the body tends to accumulate in the kidneys and bones. The maximum safe dose of protactinium in the human body is 0.03 [[Curie (unit)|μCi]] that corresponds to 0.5 micrograms of <sup>231</sup>Pa. This isotope, which might be present in the air as [[aerosol]], is 2.5{{e|8}} times more toxic than [[hydrocyanic acid]].<ref name="Himiya protaktiniya" />{{page needed|date=August 2024}}

Plutonium, when entering the body through air, food or blood (e.g. a wound), mostly settles in the lungs, liver and bones with only about 10% going to other organs, and remains there for decades. The long residence time of plutonium in the body is partly explained by its poor solubility in water. Some isotopes of plutonium emit ionizing α-radiation, which damages the surrounding cells. The [[median lethal dose]] (LD<sub>50</sub>) for 30 days in dogs after intravenous injection of plutonium is 0.32 milligram per kg of body mass, and thus the lethal dose for humans is approximately 22&nbsp;mg for a person weighing 70&nbsp;kg; the amount for respiratory exposure should be approximately four times greater. Another estimate assumes that plutonium is 50 times less toxic than [[radium]], and thus permissible content of plutonium in the body should be 5&nbsp;μg or 0.3 μCi. Such amount is nearly invisible under microscope. After trials on animals, this maximum permissible dose was reduced to 0.65&nbsp;μg or 0.04 μCi. Studies on animals also revealed that the most dangerous plutonium exposure route is through inhalation, after which 5–25% of inhaled substances is retained in the body. Depending on the particle size and solubility of the plutonium compounds, plutonium is localized either in the lungs or in the [[lymphatic system]], or is absorbed in the blood and then transported to the liver and bones. Contamination via food is the least likely way. In this case, only about 0.05% of soluble and 0.01% of insoluble compounds of plutonium absorbs into blood, and the rest is excreted. Exposure of damaged skin to plutonium would retain nearly 100% of it.<ref name="Plutoniy">{{cite book|editor1=B.A. Nadykto|editor2=L.F.Timofeeva|title=Plutonium|place=Sarov|publisher=VNIIEF|year=2003|volume=1|series=Fundamental Problems|isbn=978-5-9515-0024-3}}</ref>

Using actinides in nuclear fuel, sealed radioactive sources or advanced materials such as self-glowing crystals has many potential benefits. However, a serious concern is the extremely high radiotoxicity of actinides and their migration in the environment.<ref>{{cite book|author1=M. I. Ojovan|author2=W.E. Lee|title=An Introduction to Nuclear Waste Immobilisation|publisher=Elsevier|place=Amsterdam|year=2005|url=https://books.google.com/books?id=vQkQnmo_bE0C|isbn=978-0-08-044462-8}}</ref> Use of chemically unstable forms of actinides in MOX and sealed radioactive sources is not appropriate by modern safety standards. There is a challenge to develop stable and durable actinide-bearing materials, which provide safe storage, use and final disposal. A key need is application of actinide solid solutions in durable crystalline host phases.<ref name=burakov />

== See also ==
* [[Actinides in the environment]]
* [[Lanthanides]]
* [[Major actinides]]
* [[Minor actinides]]
* [[transuranium|Transuranics]]
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==Notes ==
{{reflist|group=notes}}

== References ==
{{reflist|30em}}

== Bibliography ==
* {{cite book|author=Golub, A. M.|title=Общая и неорганическая химия (General and Inorganic Chemistry)|year=1971|volume=2}}
* {{Greenwood&Earnshaw2nd}}
* {{cite book|author=Myasoedov, B.|title=Analytical chemistry of transplutonium elements|place=Moscow|publisher=Nauka|year=1972|isbn=978-0-470-62715-0|title-link=Transuranium element}}

== External links ==
{{Commons category}}
* [https://web.archive.org/web/20120220054516/http://imglib.lbl.gov/ImgLib/COLLECTIONS/BERKELEY-LAB/SEABORG-ARCHIVE/index/96B05654.html Lawrence Berkeley Laboratory image of historic periodic table by Seaborg showing actinide series for the first time]
* [https://web.archive.org/web/20120813024043/https://www.llnl.gov/str/pdfs/06_00.2.pdf#search=%22actinide%20series%22 Lawrence Livermore National Laboratory, ''Uncovering the Secrets of the Actinides'']
* [https://www.lanl.gov/media/publications/actinide-research-quarterly Los Alamos National Laboratory, ''Actinide Research Quarterly'']

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[[Category:Actinides| ]]
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