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{{Infobox francium}}
'''Francium''' is a [[chemical element]]; it has [[chemical symbol|symbol]] '''Fr''' and [[atomic number]] 87. It is extremely [[radioactive]]; its most stable isotope, francium-223 (originally called ''[[actinium]]&nbsp;K'' after the natural [[decay chain]] in which it appears), has a [[half-life]] of only 22&nbsp;minutes. It is the second-most [[electronegative|electropositive]] element, behind only [[caesium]],<!--PLEASE DO NOT CHANGE THIS; IT IS CORRECT. SEE BELOW. FRANCIUM IS VERY HEAVY, SO THE 7s ELECTRON MOVES SO FAST THAT YOU MUST CONSIDER RELATIVITY! THE EFFECT IS A STABILIZATION OF THE 7s ORBITAL, ENOUGH TO BRING THE ELECTRONEGATIVITY UP ABOVE CAESIUM'S!--> and is the [[Abundance of the chemical elements|second rarest]] naturally occurring element (after [[astatine]]). Francium's isotopes decay quickly into astatine, [[radium]], and [[radon]]. The [[electronic structure]] of a francium atom is [Rn] 7s<sup>1</sup>; thus, the element is classed as an [[alkali metal]].

As a consequence of its extreme instability, bulk francium has never been seen. Because of the general appearance of the other elements in its periodic table column, it is presumed that francium would appear as a highly reactive metal if enough could be collected together to be viewed as a bulk solid or liquid. Obtaining such a sample is highly improbable since the extreme heat of decay resulting from its short half-life would immediately vaporize any viewable quantity of the element.

Francium was discovered by [[Marguerite Perey]]<ref>{{Cite web |last=Guruge |first=Amila Ruwan |date=2023-01-25 |title=Francium |url=https://www.arhse.com/francium/ |access-date=2023-02-28 |website=Chemical and Process Engineering |language=en-US}}</ref> in France (from which the element takes its name) on January 7, 1939.<ref>{{cite journal |last=Perey |first=M. |date=October 1, 1939 |title = L'élément 87: AcK, dérivé de l'actinium |journal=Journal de Physique et le Radium |language=fr |volume=10 |issue=10 |pages=435–438 |doi=10.1051/jphysrad:019390010010043500 |issn=0368-3842 |url=https://hal.archives-ouvertes.fr/jpa-00233698/document}}</ref> Before its discovery, francium was referred to as ''[[Mendeleev's predicted elements|eka]]-[[caesium]]'' or ''ekacaesium'' because of its conjectured existence below caesium in the periodic table. It was the last element first discovered in nature, rather than by synthesis.{{NoteTag|Some synthetic elements, like [[technetium]] and [[plutonium]], have later been found in nature.}} Outside the laboratory, francium is extremely rare, with trace amounts found in [[uranium]] ores, where the [[isotope]] francium-223 (in the family of uranium-235) continually forms and decays. As little as {{convert|1|oz|g}} exists at any given time throughout the [[Earth's crust]]; aside from francium-223 and francium-221, its other isotopes are entirely synthetic. The largest amount produced in the laboratory was a cluster of more than 300,000 atoms.<ref name="chemnews" />

==Characteristics==
Francium is one of the most unstable of the naturally occurring elements: its longest-lived isotope, francium-223, has a [[half-life]] of only 22&nbsp;minutes. The only comparable element is [[astatine]], whose most stable natural isotope, astatine-219 (the alpha daughter of francium-223), has a half-life of 56&nbsp;seconds, although synthetic astatine-210 is much longer-lived with a half-life of 8.1&nbsp;hours.<ref name="andyscouse" /> All isotopes of francium decay into astatine, [[radium]], or [[radon]].<ref name="andyscouse">{{cite web | last = Price | first = Andy| title = Francium | date = December 20, 2004| url = http://www.andyscouse.com/pages/francium.htm | access-date = February 19, 2012}}</ref> Francium-223 also has a shorter half-life than the longest-lived isotope known of each element up to and including element 105, [[dubnium]].<ref name="CRC2006">{{cite book |year =2006 |title = CRC Handbook of Chemistry and Physics |volume = 4 |page= 12 |publisher = CRC |isbn= 978-0-8493-0474-3}}</ref>

Francium is an alkali metal whose chemical properties mostly resemble those of caesium.<ref name="CRC2006" /> A heavy element with a single [[valence electron]],<ref>{{cite web| last = Winter| first = Mark| title = Electron Configuration| work = Francium| publisher = The University of Sheffield| url = http://www.webelements.com/webelements/elements/text/Fr/eneg.html| access-date = April 18, 2007}}</ref> it has the highest [[equivalent weight]] of any element.<ref name="CRC2006" /> Liquid francium—if created—should have a [[surface tension]] of 0.05092&nbsp;[[newton (unit)|N]]/m at its melting point.<ref name="Kozhitov">{{cite journal |last1 = Kozhitov| first1 = L. V.| last2=Kol'tsov|first2=V. B. |last3=Kol'tsov|first3=A. V.| s2cid = 97764887| title = Evaluation of the Surface Tension of Liquid Francium|journal = Inorganic Materials | volume = 39| issue = 11 |pages = 1138–1141 |year = 2003 |doi = 10.1023/A:1027389223381}}</ref> Francium's melting point was estimated to be around {{convert|8.0|C|F}};<ref name="L&P">{{cite book |title=Analytical Chemistry of Technetium, Promethium, Astatine, and Francium |first1=Avgusta Konstantinovna |last1=Lavrukhina |first2=Aleksandr Aleksandrovich |last2=Pozdnyakov |year=1970 |publisher=Ann Arbor–Humphrey Science Publishers |others=Translated by R. Kondor |isbn=978-0-250-39923-9 |page=269}}</ref> a value of {{convert|27|C|F}} is also often encountered.<ref name="CRC2006" /> The melting point is uncertain because of the element's extreme rarity and [[radioactivity]]; a different extrapolation based on [[Dmitri Mendeleev]]'s method gave {{convert|20|±|1.5|C|F}}. A calculation based on the melting temperatures of binary ionic crystals gives {{convert|24.861|±|0.517|C|F}}.<ref>{{cite journal |last=Oshchapovskii |first=V. V. |date=2014 |title =A New Method of Calculation of the Melting Temperatures of Crystals of Group 1A Metal Halides and Francium Metal |journal=Russian Journal of Inorganic Chemistry |language=en |volume=59 |issue=6 |pages=561–567 |doi=10.1134/S0036023614060163 |s2cid=98622837 |issn= |url=}}</ref> The estimated boiling point of {{convert|620|C|F}} is also uncertain; the estimates {{convert|598|C|F}} and {{convert|677|C|F}}, as well as the extrapolation from Mendeleev's method of {{convert|640|C|F}}, have also been suggested.<ref name="L&P" /><ref name="Kozhitov" /> The density of francium is expected to be around 2.48&nbsp;g/cm<sup>3</sup> (Mendeleev's method extrapolates 2.4&nbsp;g/cm<sup>3</sup>).<ref name="L&P" /> Another calculation gives a much higher value of 3.57&nbsp;g/cm<sup>3</sup>.<ref name="Andres"/> Francium is predicted to have a [[bulk modulus]] of 2.1–2.6&nbsp;GPa.<ref name="Andres">{{cite journal | last1=Robles-Navarro | first1=Andres | last2=Schwerdtfeger | first2=Peter | title=Accurate relativistic density functional calculations for the solid-state of metallic francium | journal=Physical Chemistry Chemical Physics | volume=28 | issue=11 | date=2026 | issn=1463-9076 | doi=10.1039/D5CP04548G | pages=6703–6710 }}</ref>

{{anchor|electronegativity}}[[Linus Pauling]] estimated the [[electronegativity]] of francium at 0.7 on the [[Pauling scale]], the same as caesium;<ref>{{cite book |last = Pauling | first = Linus | title = The Nature of the Chemical Bond |edition = Third | author-link = Linus Pauling |publisher = Cornell University Press |year = 1960 | isbn = 978-0-8014-0333-0 |page = 93}}</ref> the value for caesium has since been refined to 0.79, but there are no experimental data to allow a refinement of the value for francium.<ref>{{cite journal |author = Allred, A. L. |year = 1961 |journal= J. Inorg. Nucl. Chem.|volume= 17 |issue= 3–4 |pages= 215–221 |title= Electronegativity values from thermochemical data |doi= 10.1016/0022-1902(61)80142-5}}</ref> Francium has a slightly higher [[ionization energy]] than caesium,<ref>{{cite journal|author = Andreev, S.V.|author2 = Letokhov, V.S.|author3 = Mishin, V.I.|title = Laser resonance photoionization spectroscopy of Rydberg levels in Fr|journal = [[Physical Review Letters]]|date = 1987|volume = 59|pages = 1274–76|doi = 10.1103/PhysRevLett.59.1274|pmid=10035190|bibcode=1987PhRvL..59.1274A|issue = 12}}</ref> 392.811(4)&nbsp;kJ/mol as opposed to 375.7041(2)&nbsp;kJ/mol for caesium, as would be expected from [[Relativistic quantum chemistry|relativistic effects]], and this would imply that caesium is the less electronegative of the two. Francium should also have a higher [[electron affinity]] than caesium and the Fr<sup>−</sup> ion should be more [[polarizability|polarizable]] than the Cs<sup>−</sup> ion.<ref name="Thayer">{{cite book |last1=Thayer |first1=John S. |title=Relativistic Methods for Chemists|chapter=Chap.10 Relativistic Effects and the Chemistry of the Heavier Main Group Elements |date=2010 |page=81 |isbn=978-1-4020-9975-5 |publisher=Springer |doi=10.1007/978-1-4020-9975-5_2}}</ref>

==Compounds==
As a result of francium's instability, its salts are only known to a small extent. Francium [[coprecipitation|coprecipitates]] with several caesium [[salt (chemistry)|salts]], such as [[caesium perchlorate]], which results in small amounts of francium perchlorate. This coprecipitation can be used to isolate francium, by adapting the radiocaesium coprecipitation method of [[Lawrence E. Glendenin]] and C. M. Nelson. It will additionally coprecipitate with many other caesium salts, including the [[iodate]], the [[picrate]], the [[tartrate]] (also [[rubidium]] tartrate), the [[chloroplatinate]], and the [[silicotungstate]]. It also coprecipitates with [[silicotungstic acid]], and with [[perchloric acid]], without another alkali metal as a [[carrier (chemistry)|carrier]], which leads to other methods of separation.<ref>{{cite journal |last= Hyde |first= E. K. |title= Radiochemical Methods for the Isolation of Element 87 (Francium) |journal= [[J. Am. Chem. Soc.]] |date= 1952 |volume= 74 |issue= 16 |pages= 4181–4184 |doi= 10.1021/ja01136a066|bibcode= 1952JAChS..74.4181H |hdl= 2027/mdp.39015086483156 |s2cid= 95854270 |hdl-access= free}}</ref><ref name="francrad">E. N K. Hyde ''Radiochemistry of Francium'', Subcommittee on Radiochemistry, National Academy of Sciences-National Research Council; available from the Office of Technical Services, Dept. of Commerce, 1960.</ref>

===Francium perchlorate===
Francium perchlorate is produced by the reaction of [[francium chloride]] and [[sodium perchlorate]]. The francium perchlorate [[coprecipitation|coprecipitates]] with [[caesium perchlorate]].<ref name="francrad" /> This coprecipitation can be used to isolate francium, by adapting the radiocaesium coprecipitation method of [[Lawrence E. Glendenin]] and C. M. Nelson. However, this method is unreliable in separating [[thallium]], which also coprecipitates with caesium.<ref name="francrad" /> Francium perchlorate's [[entropy]] is expected to be 42.7&nbsp;[[Entropy unit|e.u]]<ref name="L&P" /> (178.7 J mol<sup>−1</sup> K<sup>−1</sup>).

===Francium halides===
Francium halides are all soluble in water and are expected to be white solids. They are expected to be produced by the reaction of the corresponding [[halogen]]s. For example, francium chloride would be produced by the reaction of francium and [[chlorine]]. Francium chloride has been studied as a pathway to separate francium from other elements, by using the high [[vapour pressure]] of the compound, although francium fluoride would have a higher vapour pressure.<ref name="L&P" />

===Other compounds===
Francium nitrate, sulfate, hydroxide, carbonate, acetate, and oxalate, are all soluble in water, while the [[iodate]], [[picrate]], [[tartrate]], [[Chloroplatinic acid|chloroplatinate]], and [[silicotungstate]] are insoluble. The insolubility of these compounds are used to extract francium from other radioactive products, such as [[zirconium]], [[niobium]], [[molybdenum]], [[tin]], [[antimony]], the method mentioned in the section above.<ref name="L&P" /> Francium oxide is believed to disproportionate to the peroxide and francium metal.<ref>{{cite report|page=9|id=UCRL-409|url=https://escholarship.org/uc/item/8056g18b|title=Low Mass Francium and Emanation Isotopes of High Alpha Stability|first1=E.&nbsp;K.|last1=Hyde|first2=A.|last2=Ghiorso|author-link2=Albert Ghiorso|first3=G.&nbsp;T.|last3=Seaborg|author-link3=Glenn Seaborg|date=10 Oct 1949|location=Berkeley, CA|institution=[[UC Radiation Laboratory]]}}</ref>  The CsFr molecule is predicted to have the heavier element (francium) at the negative end of the dipole, unlike all known heterodiatomic alkali metal molecules. Francium [[superoxide]] (FrO<sub>2</sub>) is expected to have a more [[covalent]] character than its lighter [[congener (chemistry)|congeners]]; this is attributed to the 6p electrons in francium being more involved in the francium–oxygen bonding.<ref name="Thayer" /> The relativistic destabilisation of the 6p<sub>3/2</sub> spinor may make francium compounds in oxidation states higher than +1 possible, such as [Fr<sup>V</sup>F<sub>6</sub>]<sup>−</sup>; but this has not been experimentally confirmed.<ref>{{cite journal |last1=Cao |first1=Chang-Su |last2=Hu |first2=Han-Shi |last3=Schwarz |first3=W. H. Eugen |last4=Li |first4=Jun |date=2022 |title=Periodic Law of Chemistry Overturns for Superheavy Elements |type=preprint |url=https://chemrxiv.org/engage/chemrxiv/article-details/63730be974b7b6d84cfdda35 |journal=[[ChemRxiv]] |volume= |issue= |pages= |doi=10.26434/chemrxiv-2022-l798p |access-date=16 November 2022|doi-access=free }}</ref>

==Isotopes==
{{main|Isotopes of francium}}

There are 37 known isotopes of francium ranging in [[atomic mass]] from 197 to 233.{{NUBASE2020|ref}} Francium has seven [[metastability|metastable]] [[nuclear isomer]]s.<ref name="CRC2006" /> Francium-223 and francium-221 are the only isotopes that occur in nature, with the former being far more common.<ref name="nostrand679">{{cite book|date = 2005|title= Francium, in Van Nostrand's Encyclopedia of Chemistry|editor-last = Considine| editor-first = Glenn D.| page= 679|location= New York| publisher = Wiley-Interscience| isbn = 978-0-471-61525-5}}</ref>

Francium-223 is the most stable isotope, with a half-life of 21.8&nbsp;minutes,<ref name="CRC2006" /> and it is highly unlikely that an isotope of francium with a longer half-life will ever be discovered or synthesized.<ref name="mcgraw" /> Francium-223 is a fifth product of the [[uranium-235]] decay series as a daughter isotope of [[actinium-227]]; [[thorium-227]] is the more common daughter.<ref name="nostrand332">{{cite book|date = 2005|title= Chemical Elements, in Van Nostrand's Encyclopedia of Chemistry|editor-last = Considine| editor-first = Glenn D.|page=332|location= New York| publisher = Wiley-Interscience| isbn = 978-0-471-61525-5}}</ref> Francium-223 then decays into radium-223 by [[beta decay]] (1.149&nbsp;MeV [[decay energy]]), with a minor (0.006%) [[alpha decay]] path to astatine-219 (5.4&nbsp;MeV decay energy).<ref>{{cite web |author=National Nuclear Data Center |date=1990 |title=Table of Isotopes decay data |url=http://ie.lbl.gov/toi/nuclide.asp?iZA=870223 |publisher=[[Brookhaven National Laboratory]] |access-date=April 4, 2007 |archive-url=https://web.archive.org/web/20061031212436/http://ie.lbl.gov/toi/nuclide.asp?iZA=870223 |archive-date=October 31, 2006}}</ref>

Francium-221 has a half-life of 4.8&nbsp;minutes.<ref name="CRC2006" /> It is the ninth product of the [[neptunium]] decay series as a daughter isotope of [[actinium-225]].<ref name="nostrand332" /> Francium-221 then decays into astatine-217 by alpha decay (6.457&nbsp;MeV decay energy).<ref name="CRC2006" /> Although all primordial <sup>237</sup>Np is [[extinct radionuclide|extinct]], the neptunium decay series continues to exist naturally in tiny traces due to (n,2n) knockout reactions in natural <sup>238</sup>U.<ref name=4n1>{{cite journal |last1=Peppard |first1=D. F. |last2=Mason |first2=G. W. |last3=Gray |first3=P. R. |last4=Mech |first4=J. F. |title=Occurrence of the (4n + 1) series in nature |journal=Journal of the American Chemical Society |date=1952 |volume=74 |issue=23 |pages=6081–6084 |doi=10.1021/ja01143a074 |bibcode=1952JAChS..74.6081P |url=https://digital.library.unt.edu/ark:/67531/metadc172698/m2/1/high_res_d/metadc172698.pdf }}</ref> Francium-222, with a half-life of 14&nbsp;minutes, may be produced as a result of the beta decay of natural [[radon-222]]; this process has nonetheless not yet been observed,<ref name="bellidecay">{{cite journal |last1=Belli |first1=P. |last2=Bernabei |first2=R. |last3=Danevich |first3=F. A. |last4=Incicchitti |first4=A. |last5=Tretyak |first5=V. I. |display-authors=3 |title=Experimental searches for rare alpha and beta decays |journal=European Physical Journal A |date=2019 |volume=55 |issue=8 |pages=140–1–140–7 |doi=10.1140/epja/i2019-12823-2 |issn=1434-601X |arxiv=1908.11458|bibcode=2019EPJA...55..140B |s2cid=201664098 }}</ref> and it is unknown whether this process is energetically possible.{{NoteTag|AME2020 gives <sup>222</sup>Rn a lower mass than <sup>222</sup>Fr,{{AME2020 II|ref}} which would forbid single beta decay, though it is possible within the given error margin and is explicitly predicted by Belli et al.<ref name=BelliBaF2>{{cite journal |last1=Belli |first1=P. |last2=Bernabei |first2=R. |last3=Cappella |first3=C. |last4=Caracciolo |first4=V. |last5=Cerulli |first5=R. |last6=Danevich |first6=F.A. |last7=Di Marco |first7=A. |last8=Incicchitti |first8=A. |last9=Poda |first9=D.V. |last10=Polischuk |first10=O.G. |last11=Tretyak |first11=V.I. |title=Investigation of rare nuclear decays with BaF<sub>2</sub> crystal scintillator contaminated by radium |date=2014 |journal=European Physical Journal A |volume=50 |issue=9 |pages=134–143 |doi=10.1140/epja/i2014-14134-6 |arxiv=1407.5844|bibcode=2014EPJA...50..134B |s2cid=118513731 }}</ref>}}

The least stable [[ground state]] isotope is francium-215, with a half-life of 90&nbsp;ns:{{NUBASE2020|ref}} it undergoes a 9.54&nbsp;MeV alpha decay to astatine-211.<ref name="CRC2006" />

==Applications==
Due to its instability and rarity, there are no commercial applications for francium.<ref>{{cite web| last = Winter| first = Mark| title = Uses| work = Francium| publisher = The University of Sheffield|url = http://www.webelements.com/webelements/elements/text/Fr/uses.html| access-date = March 25, 2007}}</ref><ref name="nbb">{{cite book| last = Emsley|url=https://books.google.com/books?id=Yhi5X7OwuGkC&pg=PA151| first = John| title = Nature's Building Blocks| publisher = Oxford University Press| date = 2001| location = Oxford| pages = 151–153| isbn = 978-0-19-850341-5}}</ref><ref name="elemental">{{cite web| last = Gagnon| first = Steve| title = Francium| publisher = Jefferson Science Associates, LLC| url = http://education.jlab.org/itselemental/ele087.html| access-date = April 1, 2007| archive-date = March 31, 2007| archive-url = https://web.archive.org/web/20070331235139/http://education.jlab.org/itselemental/ele087.html| url-status = dead}}</ref><ref name="nostrand332" /> It has been used for research purposes in the fields of [[chemistry]]<ref name="bio">{{cite journal | last1 = Haverlock |first1 = T. J. |pmid = 12553788 |doi= 10.1021/ja0255251|title = Selectivity of calix[4]arene-bis(benzocrown-6) in the complexation and transport of francium ion |journal = J Am Chem Soc|date = 2003|volume=125|pages=1126–7| last2 = Mirzadeh| first2 = S.|last3 = Moyer| first3 = B. A.|issue = 5|bibcode = 2003JAChS.125.1126H }}</ref>
and of [[atom|atomic structure]]. Its use as a potential diagnostic aid for various [[cancer]]s has also been explored,<ref name="andyscouse" /> but this application has been deemed impractical.<ref name="nbb" />

Francium's ability to be synthesized, trapped, and cooled, along with its relatively simple [[atom|atomic structure]], has made it the subject of specialized [[spectroscopy]] experiments. These experiments have led to more specific information regarding [[energy level]]s and the [[coupling constant]]s between [[subatomic particle]]s.<ref>{{cite journal| last = Gomez| first = E.|last2=Orozco |first2= L A |last3=Sprouse |first3= G D | s2cid = 15917603| title = Spectroscopy with trapped francium: advances and perspectives for weak interaction studies| journal = Rep. Prog. Phys.| volume = 69| issue = 1| pages = 79–118| date = November 7, 2005|doi = 10.1088/0034-4885/69/1/R02|bibcode = 2006RPPh...69...79G}}</ref> Studies on the light emitted by [[Optical tweezers|laser-trapped]] francium-210 ions have provided accurate data on transitions between atomic energy levels which are fairly similar to those predicted by [[quantum mechanics|quantum theory]].<ref>{{cite journal|last = Peterson|first = I.|title = Creating, cooling, trapping francium atoms|journal = Science News|date = May 11, 1996|url = http://www.sciencenews.org/pages/pdfs/data/1996/149-19/14919-06.pdf|access-date = September 11, 2001|volume = 149|issue = 19|page = 294|doi = 10.2307/3979560|jstor = 3979560|archive-date = July 27, 2020|archive-url = https://web.archive.org/web/20200727014700/https://www.sciencenews.org/pages/pdfs/data/1996/149-19/14919-06.pdf}}</ref> Francium is a prospective candidate for searching for [[CP violation]].<ref>{{cite journal | last1=Osika | first1=Yuliya | last2=Meniailava | first2=Darya | last3=Shundalau | first3=Maksim | title=Relativistic coupled cluster study on the spectroscopic and radiative properties of the KFr molecule and modeling of the transport properties of potassium–francium dilute gas medium | journal=Journal of Quantitative Spectroscopy and Radiative Transfer | publisher=Elsevier BV | volume=321 | year=2024 | issn=0022-4073 | doi=10.1016/j.jqsrt.2024.108996 | article-number=108996| bibcode=2024JQSRT.32108996O }}</ref>

==History==
As early as 1870, chemists thought that there should be an alkali metal beyond [[caesium]], with an atomic number of 87.<ref name="andyscouse" /> It was then referred to by the provisional name ''[[Mendeleev's predicted elements|eka-caesium]]''.<ref name="chemeducator">Adloff, Jean-Pierre; Kaufman, George B. (September 25, 2005). [http://chemeducator.org/sbibs/s0010005/spapers/1050387gk.htm Francium (Atomic Number 87), the Last Discovered Natural Element] {{Webarchive|url=https://web.archive.org/web/20130604212956/http://chemeducator.org/sbibs/s0010005/spapers/1050387gk.htm |date=June 4, 2013}} . ''The Chemical Educator'' '''10''' (5). Retrieved on March 26, 2007.</ref>

===Erroneous and incomplete discoveries===
In 1914, [[Stefan Meyer (physicist)|Stefan Meyer]], Viktor F. Hess, and [[Friedrich Paneth]] (working in Vienna) made measurements of alpha radiation from various substances, including <sup>227</sup>Ac. They observed the possibility of a minor alpha branch of this nuclide, though follow-up work could not be done due to the outbreak of [[World War I]]. Their observations were not precise and sure enough for them to announce the discovery of element 87, though it is likely that they did indeed observe the decay of <sup>227</sup>Ac to <sup>223</sup>Fr.<ref name=chemeducator/>

Soviet chemist [[Dmitry Dobroserdov]] was the first scientist to claim to have found eka-caesium, or francium. In 1925, he observed weak radioactivity in a sample of [[potassium]], another alkali metal, and incorrectly concluded that eka-caesium was contaminating the sample (the radioactivity from the sample was from the naturally occurring potassium radioisotope, [[potassium-40]]).<ref name="fontani">{{cite conference| first = Marco| last = Fontani |author-link= Marco Fontani | title = The Twilight of the Naturally-Occurring Elements: Moldavium (Ml), Sequanium (Sq) and Dor (Do)| book-title = International Conference on the History of Chemistry| pages = 1–8| date = September 10, 2005| location = Lisbon|url = http://5ichc-portugal.ulusofona.pt/uploads/PaperLong-MarcoFontani.doc| archive-url = https://web.archive.org/web/20060224090117/http://5ichc-portugal.ulusofona.pt/uploads/PaperLong-MarcoFontani.doc|archive-date=February 24, 2006|access-date = April 8, 2007}}</ref> He then published a thesis on his predictions of the properties of eka-caesium, in which he named the element ''russium'' after his home country.<ref name="vanderkroft">{{cite web| last = Van der Krogt| first = Peter| title = Francium| work = Elementymology & Elements Multidict| date = January 10, 2006| url = http://elements.vanderkrogt.net/element.php?sym=Fr| access-date = April 8, 2007}}</ref> Shortly thereafter, Dobroserdov began to focus on his teaching career at the Polytechnic Institute of [[Odesa]], and he did not pursue the element further.<ref name="fontani" />

The following year, English chemists [[Gerald J. F. Druce]] and [[Frederick H. Loring]] analyzed [[X-ray]] photographs of [[manganese(II) sulfate]].<ref name="vanderkroft" /> They observed spectral lines which they presumed to be of eka-caesium. They announced their discovery of element 87 and proposed the name ''alkalinium'', as it would be the heaviest alkali metal.<ref name="fontani" />

In 1930, [[Fred Allison]] of the [[Alabama Polytechnic Institute]] claimed to have discovered element 87 (in addition to 85) when analyzing [[pollucite]] and [[lepidolite]] using his [[magneto-optic effect|magneto-optical]] machine. Allison requested that it be named ''virginium'' after his home state of [[Virginia]], along with the symbols Vi and Vm<!--along with them how? and proposed the symbols Vi and Vm?-->.<ref name="vanderkroft" /><ref>{{cite magazine| title = Alabamine & Virginium| magazine = [[Time (magazine)|Time]] | date = February 15, 1932|url = http://www.time.com/time/magazine/article/0,9171,743159,00.html|archive-url = https://web.archive.org/web/20070930015028/http://www.time.com/time/magazine/article/0,9171,743159,00.html|archive-date = September 30, 2007| access-date = April 1, 2007}}</ref> In 1934, H.G. MacPherson of [[University of California, Berkeley|UC Berkeley]] disproved the effectiveness of Allison's device and the validity of his discovery.<ref>{{cite journal| last = MacPherson| first = H. G.| title = An Investigation of the Magneto-Optic Method of Chemical Analysis| journal = Physical Review| volume = 47| issue = 4| pages = 310–315|date=1934|doi = 10.1103/PhysRev.47.310|bibcode = 1935PhRv...47..310M}}</ref>

In 1936, Romanian physicist [[Horia Hulubei]] and his French colleague [[Yvette Cauchois]] also analyzed pollucite, this time using their high-resolution X-ray apparatus.<ref name="fontani" /> They observed several weak emission lines, which they presumed to be those of element 87. Hulubei and Cauchois reported their discovery and proposed the name ''moldavium'', along with the symbol Ml, after [[Moldavia]], the Romanian province where Hulubei was born.<ref name="vanderkroft" /> In 1937, Hulubei's work was criticized by American physicist [[F. H. Hirsh Jr.]], who rejected Hulubei's research methods. Hirsh was certain that eka-caesium would not be found in nature, and that Hulubei had instead observed [[mercury (element)|mercury]] or [[bismuth]] X-ray lines. Hulubei insisted that his X-ray apparatus and methods were too accurate to make such a mistake. Because of this, [[Jean Baptiste Perrin]], [[Nobel Prize]] winner and Hulubei's mentor, endorsed moldavium as the true eka-caesium over [[Marguerite Perey]]'s recently discovered francium. Perey took pains to be accurate and detailed in her criticism of Hulubei's work, and finally she was credited as the sole discoverer of element 87.<ref name="fontani" /> All other previous purported discoveries of element 87 were ruled out due to francium's very limited half-life.<ref name="vanderkroft" />

===Perey's analysis===
Eka-caesium was discovered on January 7, 1939, by [[Marguerite Perey]] of the [[Curie Institute (Paris)|Curie Institute]] in Paris,<ref name=chemeducator/> when she purified a sample of [[actinium]]-227 which had been reported to have a decay energy of 220&nbsp;keV. Perey noticed decay particles with an energy level below 80&nbsp;keV. Perey thought this decay activity might have been caused by a previously unidentified decay product, one which was separated during purification, but emerged again out of the pure actinium-227. Various tests eliminated the possibility of the unknown element being [[thorium]], radium, [[lead]], bismuth, or [[thallium]]. The new product exhibited chemical properties of an alkali metal (such as coprecipitating with caesium salts), which led Perey to believe that it was element 87, produced by the [[alpha decay]] of actinium-227.<ref name="chemeducator" /> Perey then attempted to determine the proportion of [[beta decay]] to alpha decay in actinium-227. Her first test put the alpha branching at 0.6%, a figure which she later revised to 1%.<ref name="mcgraw" />

Perey named the new isotope ''actinium-K'' (it is now referred to as francium-223)<ref name="chemeducator" /> and in 1946, she proposed the name ''catium'' (Cm) for her newly discovered element, as she believed it to be the most [[electronegativity|electropositive]] [[cation]] of the elements. [[Irène Joliot-Curie]], one of Perey's supervisors, opposed the name due to its connotation of ''cat'' rather than ''cation''; furthermore, the symbol coincided with that which had since been assigned to [[curium]].<ref name="chemeducator" /> Perey then suggested ''francium'', after France. This name was officially adopted by the [[International Union of Pure and Applied Chemistry]] (IUPAC) in 1949,<ref name="andyscouse" /> becoming the second element after [[gallium]] to be named after France. It was assigned the symbol Fa, but it was revised to the current Fr shortly thereafter.<ref name="hackh">{{Cite book| last = Grant| first = Julius| contribution = Francium| date = 1969| title = Hackh's Chemical Dictionary| pages = 279–280| publisher = McGraw-Hill| isbn = 978-0-07-024067-4}}</ref> Francium was the last element discovered in nature, rather than synthesized, following [[hafnium]] and [[rhenium]].<ref name="chemeducator" /> Further research into francium's structure was carried out by, among others, [[Sylvain Lieberman]] and his team at [[CERN]] in the 1970s and 1980s.<ref>{{cite web
 |title       = History
 |work        = Francium
 |publisher   = [[State University of New York at Stony Brook]]
 |date        = February 20, 2007
 |url         = http://fr.physics.sunysb.edu/francium_news/history.HTM
 |access-date = March 26, 2007
 |archive-url = https://web.archive.org/web/20080515132357/http://fr.physics.sunysb.edu/francium_news/history.HTM
 |archive-date = May 15, 2008
 }}</ref>

==Occurrence==
[[File:Pichblende.jpg|thumb|This sample of [[uraninite]] contains about 100,000 atoms (3.7{{e|-17}}&nbsp;g) of francium-223 at any given time.<ref name="nbb" />|alt=A shiny gray 5-centimeter piece of matter with a rough surface.]]
<sup>223</sup>Fr is the result of the alpha decay of [[Isotopes of actinium|<sup>227</sup>Ac]] and can be found in trace amounts in [[uranium]] [[mineral]]s.<ref name="CRC2006" /> In a given sample of uranium, there is estimated to be only one francium atom for every 1 × 10<sup>18</sup> uranium atoms.<ref name="nbb" /> Only about {{convert|1|oz|g}} of francium is present naturally in the earth's crust.<ref>{{Cite book |last=Krebs |first=Robert E. |url=https://books.google.com/books?id=D7LOEAAAQBAJ&dq=%22francium%20hydroxide%22%20existence&pg=PA64 |title=The History and Use of Our Earth's Chemical Elements: A Reference Guide |date=2006-07-30 |publisher=Bloomsbury Publishing USA |isbn=978-0-313-02798-7 |language=en}}</ref>

==Production==
[[File:franciumtrap.PNG|thumb|left|A [[magneto-optical trap]], which can hold neutral francium atoms for short periods of time.<ref name="sbtrapping" />|alt=A complex experimental setup featuring a horizontal glass tube placed between two copper coils.]]
Francium can be synthesized by a [[nuclear fusion|fusion]] reaction when a gold-197 target is bombarded with a beam of oxygen-18 atoms from a [[linear accelerator]] in a process originally developed at the physics department of the [[State University of New York at Stony Brook]] in 1995.<ref name="sbproduction">{{cite web| title = Production of Francium| work = Francium| publisher = [[State University of New York at Stony Brook]]| date = February 20, 2007| url = http://fr.physics.sunysb.edu/francium_news/production.HTM| access-date = March 26, 2007| archive-url = https://web.archive.org/web/20071012010344/http://fr.physics.sunysb.edu/francium_news/production.HTM| archive-date = October 12, 2007}}</ref> Depending on the energy of the oxygen beam, the reaction can yield francium isotopes with masses of 209, 210, and 211.

:<sup>197</sup>Au + <sup>18</sup>O → <sup>209</sup>Fr + 6 n
:<sup>197</sup>Au + <sup>18</sup>O → <sup>210</sup>Fr + 5 n
:<sup>197</sup>Au + <sup>18</sup>O → <sup>211</sup>Fr + 4 n
{{multiple image
| align = right
| image1 = Francium (200,000 francium atoms in a magneto-optical trap).jpg
| width1 = {{#expr:150*150/165 round 0}}
| alt1 = A round ball of red light surrounded by a green glow
| caption1 = Image of light emitted by a sample of 200,000 francium atoms in a magneto-optical trap
| image2 = Fr,87.jpg
| width2 = 150
| alt2 = A small white spot in the middle surrounded by a red circle. There is a yellow ring outside the red circle, a green circle beyond the yellow ring and a blue circle surrounding all the other circles.
| caption2 = Heat image of 200,000 francium atoms in a magneto-optical trap, around 100 attograms
| footer = 
}}
The francium atoms leave the gold target as ions, which are neutralized by collision with [[yttrium]] and then isolated in a [[magneto-optical trap]] (MOT) in a gaseous unconsolidated state.<ref name="sbtrapping">{{cite web| title = Cooling and Trapping| work = Francium| publisher = [[State University of New York at Stony Brook]]| date = February 20, 2007| url = http://fr.physics.sunysb.edu/francium_news/trapping.HTM| access-date = May 1, 2007| archive-url = https://web.archive.org/web/20071122170110/http://fr.physics.sunysb.edu/francium_news/trapping.HTM| archive-date = November 22, 2007}}</ref> Although the atoms only remain in the trap for about 30 seconds before escaping or undergoing nuclear decay, the process supplies a continual stream of fresh atoms. The result is a [[steady state]] containing a fairly constant number of atoms for a much longer time.<ref name="sbtrapping" /> The original apparatus could trap up to a few thousand atoms, while a later improved design could trap over 300,000 at a time.<ref name="chemnews">{{cite journal|url=http://pubs.acs.org/cen/80th/francium.html|title=Francium|journal=Chemical and Engineering News|date=2003|first=Luis A. |last=Orozco |volume=81|issue=36|page=159|doi=10.1021/cen-v081n036.p159|url-access=subscription}}</ref> Sensitive measurements of the light emitted and absorbed by the trapped atoms provided the first experimental results on various transitions between atomic energy levels in francium. Initial measurements show very good agreement between experimental values and calculations based on quantum theory. The research project using this production method relocated to [[TRIUMF]] in 2012, where over 10<sup>6</sup> francium atoms have been held at a time, including large amounts of <sup>209</sup>Fr in addition to <sup>207</sup>Fr and <sup>221</sup>Fr.<ref>{{cite report |url= https://www.osti.gov/servlets/purl/1214938 |title= Project Closeout Report: Francium Trapping Facility at TRIUMF |publisher= U.S. Department of Energy |date= September 30, 2014 |doi= 10.2172/1214938 |osti=1214938 |last1= Orozco |first1= Luis A.}}</ref><ref>{{cite journal |journal= Journal of Instrumentation |title= Commissioning of the Francium Trapping Facility at TRIUMF |first1= M |last1= Tandecki |first2= J. |last2= Zhang |first3= R. |last3= Collister |first4= S. |last4= Aubin |first5= J. A. |last5= Behr |first6= E. |last6= Gomez |first7= G. |last7= Gwinner |first8= L. A. |last8= Orozco |first9= M. R. |last9= Pearson |s2cid= 15501597 |volume= 8 |issue= 12 |article-number= 12006 |year= 2013 |doi= 10.1088/1748-0221/8/12/P12006 |arxiv= 1312.3562 |bibcode= 2013JInst...8P2006T}}</ref>

Other synthesis methods include bombarding radium with neutrons, and bombarding thorium with protons, [[deuterium|deuterons]], or [[helium]] [[ion]]s.<ref name="mcgraw">{{Cite book| contribution = Francium| date = 2002| title = McGraw-Hill Encyclopedia of Science & Technology| volume = 7| pages = [https://archive.org/details/mcgrawhillencycl165newy/page/493 493–494]| publisher = McGraw-Hill Professional| isbn = 978-0-07-913665-7| title-link = McGraw-Hill Encyclopedia of Science & Technology}}</ref>

<sup>223</sup>Fr can also be isolated from samples of its parent <sup>227</sup>Ac, the francium being milked via elution with NH<sub>4</sub>Cl–CrO<sub>3</sub> from an actinium-containing cation exchanger and purified by passing the solution through a [[silicon dioxide]] compound loaded with [[barium sulfate]].<ref>{{Ullmann | first1=Cornelius |last1=Keller |first2=Walter |last2=Wolf |first3=Jashovam |last3=Shani | title = Radionuclides, 2. Radioactive Elements and Artificial Radionuclides | doi = 10.1002/14356007.o22_o15}}</ref>

In 1996, the Stony Brook group trapped 3000 atoms in their MOT, which was enough for a video camera to capture the light given off by the atoms as they fluoresce.<ref name="chemnews" /> Francium has not been synthesized in amounts large enough to weigh.<ref name="andyscouse" /><ref name="nbb" /><ref name="losalamos">{{cite web | title = Francium |publisher = Los Alamos National Laboratory |year = 2011 |url = http://periodic.lanl.gov/87.shtml |access-date = February 19, 2012}}</ref>

== Notes ==
{{NoteFoot}}

== References ==
{{Reflist}}

== External links ==
* [http://www.periodicvideos.com/videos/087.htm Francium] at ''[[The Periodic Table of Videos]]'' (University of Nottingham)
* [http://www.webelements.com/webelements/elements/text/Fr/index.html WebElements.com – Francium]
* [https://web.archive.org/web/19981203125538/http://fr.physics.sunysb.edu/francium_news/frconten.htm Stony Brook University Physics Dept.]
* [[Eric Scerri|Scerri, Eric]] (2013). ''A Tale of Seven Elements'', Oxford University Press, Oxford, {{ISBN|9780195391312}}

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