Template:Short description Template:Pp-vand Template:Use mdy dates Template:Infobox francium Francium is a chemical element; it has symbol Fr and atomic number 87. It is extremely radioactive; its most stable isotope, francium-223 (originally called actinium K after the natural decay chain in which it appears), has a half-life of only 22 minutes. It is the second-most electropositive element, behind only caesium, and is the 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] 7s1; 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>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> in France (from which the element takes its name) on January 7, 1939.<ref>Template:Cite journal</ref> Before its discovery, francium was referred to as 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.Template:NoteTag 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 Template:Convert 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" />
CharacteristicsEdit
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 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 seconds, although synthetic astatine-210 is much longer-lived with a half-life of 8.1 hours.<ref name="andyscouse" /> All isotopes of francium decay into astatine, radium, or radon.<ref name="andyscouse">{{#invoke:citation/CS1|citation |CitationClass=web }}</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">Template:Cite book</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>{{#invoke:citation/CS1|citation |CitationClass=web }}</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 N/m at its melting point.<ref name="Kozhitov">Template:Cite journal</ref> Francium's melting point was estimated to be around Template:Convert;<ref name="L&P">Template:Cite book</ref> a value of Template:Convert 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 Template:Convert. A calculation based on the melting temperatures of binary ionic crystals gives Template:Convert.<ref>Template:Cite journal</ref> The estimated boiling point of Template:Convert is also uncertain; the estimates Template:Convert and Template:Convert, as well as the extrapolation from Mendeleev's method of Template:Convert, have also been suggested.<ref name="L&P" /><ref name="Kozhitov" /> The density of francium is expected to be around 2.48 g/cm3 (Mendeleev's method extrapolates 2.4 g/cm3).<ref name="L&P" />
Template:AnchorLinus Pauling estimated the electronegativity of francium at 0.7 on the Pauling scale, the same as caesium;<ref>Template:Cite book</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>Template:Cite journal</ref> Francium has a slightly higher ionization energy than caesium,<ref>Template:Cite journal</ref> 392.811(4) kJ/mol as opposed to 375.7041(2) kJ/mol for caesium, as would be expected from 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− ion should be more polarizable than the Cs− ion.<ref name="Thayer">Template:Cite book</ref>
CompoundsEdit
As a result of francium's instability, its salts are only known to a small extent. Francium coprecipitates with several caesium 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, which leads to other methods of separation.<ref>Template:Cite journal</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 perchlorateEdit
Francium perchlorate is produced by the reaction of francium chloride and sodium perchlorate. The francium perchlorate 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 e.u<ref name="L&P" /> (178.7 J mol−1 K−1).
Francium halidesEdit
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 halogens. 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 compoundsEdit
Francium nitrate, sulfate, hydroxide, carbonate, acetate, and oxalate, are all soluble in water, while the iodate, picrate, tartrate, 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>Template:Cite report</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 (FrO2) is expected to have a more covalent character than its lighter 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 6p3/2 spinor may make francium compounds in oxidation states higher than +1 possible, such as [FrVF6]−; but this has not been experimentally confirmed.<ref>Template:Cite journal</ref>
IsotopesEdit
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There are 37 known isotopes of francium ranging in atomic mass from 197 to 233.Template:NUBASE2020 Francium has seven metastable nuclear isomers.<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">Template:Cite book</ref>
Francium-223 is the most stable isotope, with a half-life of 21.8 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">Template:Cite book</ref> Francium-223 then decays into radium-223 by beta decay (1.149 MeV decay energy), with a minor (0.006%) alpha decay path to astatine-219 (5.4 MeV decay energy).<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
Francium-221 has a half-life of 4.8 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 MeV decay energy).<ref name="CRC2006" /> Although all primordial 237Np is extinct, the neptunium decay series continues to exist naturally in tiny traces due to (n,2n) knockout reactions in natural 238U.<ref name=4n1>Template:Cite journal</ref> Francium-222, with a half-life of 14 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">Template:Cite journal</ref> and it is unknown whether this process is energetically possible.Template:NoteTag
The least stable ground state isotope is francium-215, with a half-life of 90 ns:Template:NUBASE2020 it undergoes a 9.54 MeV alpha decay to astatine-211.<ref name="CRC2006" />
ApplicationsEdit
Due to its instability and rarity, there are no commercial applications for francium.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name="nbb">Template:Cite book</ref><ref name="elemental">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name="nostrand332" /> It has been used for research purposes in the fields of chemistry<ref name="bio">Template:Cite journal</ref> and of atomic structure. Its use as a potential diagnostic aid for various cancers 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 atomic structure, has made it the subject of specialized spectroscopy experiments. These experiments have led to more specific information regarding energy levels and the coupling constants between subatomic particles.<ref>Template:Cite journal</ref> Studies on the light emitted by laser-trapped francium-210 ions have provided accurate data on transitions between atomic energy levels which are fairly similar to those predicted by quantum theory.<ref>Template:Cite journal</ref> Francium is a prospective candidate for searching for CP violation.<ref>Template:Cite journal</ref>
HistoryEdit
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 eka-caesium.<ref name="chemeducator">Adloff, Jean-Pierre; Kaufman, George B. (September 25, 2005). Francium (Atomic Number 87), the Last Discovered Natural Element Template:Webarchive . The Chemical Educator 10 (5). Retrieved on March 26, 2007.</ref>
Erroneous and incomplete discoveriesEdit
In 1914, Stefan Meyer, Viktor F. Hess, and Friedrich Paneth (working in Vienna) made measurements of alpha radiation from various substances, including 227Ac. 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 227Ac to 223Fr.<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">Template:Cite conference</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">{{#invoke:citation/CS1|citation |CitationClass=web }}</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-optical machine. Allison requested that it be named virginium after his home state of Virginia, along with the symbols Vi and Vm.<ref name="vanderkroft" /><ref>Template:Cite magazine</ref> In 1934, H.G. MacPherson of UC Berkeley disproved the effectiveness of Allison's device and the validity of his discovery.<ref>Template:Cite journal</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 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 analysisEdit
Eka-caesium was discovered on January 7, 1939, by Marguerite Perey of the 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 keV. Perey noticed decay particles with an energy level below 80 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 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">Template:Cite book</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>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
OccurrenceEdit
223Fr is the result of the alpha decay of 227Ac and can be found in trace amounts in uranium minerals.<ref name="CRC2006" /> In a given sample of uranium, there is estimated to be only one francium atom for every 1 × 1018 uranium atoms.<ref name="nbb" /> Only about Template:Convert of francium is present naturally in the earth's crust.<ref>Template:Cite book</ref>
ProductionEdit
Francium can be synthesized by a 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">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Depending on the energy of the oxygen beam, the reaction can yield francium isotopes with masses of 209, 210, and 211.
- 197Au + 18O → 209Fr + 6 n
- 197Au + 18O → 210Fr + 5 n
- 197Au + 18O → 211Fr + 4 n
Template:Multiple image 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">{{#invoke:citation/CS1|citation |CitationClass=web }}</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">Template:Cite journal</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 106 francium atoms have been held at a time, including large amounts of 209Fr in addition to 207Fr and 221Fr.<ref>Template:Cite report</ref><ref>Template:Cite journal</ref>
Other synthesis methods include bombarding radium with neutrons, and bombarding thorium with protons, deuterons, or helium ions.<ref name="mcgraw">Template:Cite book</ref>
223Fr can also be isolated from samples of its parent 227Ac, the francium being milked via elution with NH4Cl–CrO3 from an actinium-containing cation exchanger and purified by passing the solution through a silicon dioxide compound loaded with barium sulfate.<ref>Template:Ullmann</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">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
NotesEdit
ReferencesEdit
External linksEdit
- Francium at The Periodic Table of Videos (University of Nottingham)
- WebElements.com – Francium
- Stony Brook University Physics Dept.
- Scerri, Eric (2013). A Tale of Seven Elements, Oxford University Press, Oxford, Template:ISBN
Template:Periodic table (navbox) Template:Francium compounds