Francium

Francium (pronounced /ˈfrænsiəm/), formerly known as eka-caesium and actinium K,^[1] is a chemical element that has the symbol Fr and atomic number 87. It has the lowest known electronegativity of all known elements, and is the second rarest naturally occurring element (after astatine). Francium is a highly radioactive metal that decays into astatine, radium, and radon. As an alkali metal, it has one valence electron.

Francium was discovered by Marguerite Perey in France (from which the element takes its name) in 1939. It was the last element discovered in nature, rather than synthesized.^[2] Outside the laboratory, francium is extremely rare, with trace amounts found in uranium and thorium ores, where the isotope francium-223 continually forms and decays. As little as 30 g (one ounce) exists at any given time throughout the Earth's crust; the other isotopes are entirely synthetic. The largest amount ever collected of any isotope was a cluster of 10,000 atoms (of francium-210) created as an ultracold gas at Stony Brook in 1997.^[3]

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Characteristics

Francium is less stable than any other element lighter than nobelium, element 102:^[3] its most stable isotope, francium-223, has a half-life of less than 22 minutes. By contrast, astatine, the next least stable element, has a maximum half-life of 8.5 hours.^[4] All isotopes of francium decay into either astatine, radium, or radon.^[4]

Francium is an alkali metal whose chemical properties most resemble those of caesium.^[3] A very heavy element with a single valence electron,^[5] it has the highest equivalent weight of any element.^[3] Similarly, francium has the lowest electronegativity of all the known elements at 0.7 on the Pauling scale;^[6] caesium has the second-lowest at 0.79.^[7] Liquid francium — if such a substance were to be created — should have a surface tension of 0.05092 N/m at its melting point.^[8] 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 Glendenin and 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 provides other methods of separation.^[9][10] Nearly all francium salts are water-soluble.^[11]

Applications

Due to its instability and rarity^{[12][13][14][15][16]} there are no commercial applications for francium. It has been used for research purposes in the fields of biology and of atomic structure. Its use as a potential diagnostic aid for various cancers has also been explored,^[4] but this application has been deemed impractical.^[14]

Francium's ability to be synthesized, trapped, and cooled, along with its relatively simple atomic structure have 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.^[17] 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.^[18]

History

As early as 1870, chemists thought that there should be an alkali metal beyond caesium, with an atomic number of 87.^[4] It was then referred to by the provisional name eka-caesium.^[19] Research teams attempted to locate and isolate this missing element, and at least four false claims were made that the element had been found before an authentic discovery was made.

Erroneous and incomplete discoveries

Russian chemist D. K. 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 concluded that eka-caesium was contaminating the sample.^[20] 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.^[21] Shortly thereafter, Dobroserdov began to focus on his teaching career at the Polytechnic Institute of Odessa, and he did not pursue the element further.^[20]

The following year, English chemists Gerald J. F. Druce and Frederick H. Loring analyzed X-ray photographs of manganese(II) sulfate.^[21] 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.^[20]

In 1930, Fred Allison of the Alabama Polytechnic Institute claimed to have discovered element 87 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.^[21][22] In 1934, however, H.G. MacPherson of UC Berkeley disproved the effectiveness of Allison's device and the validity of this false discovery.^[23]

In 1936, Romanian chemist Horia Hulubei and his French colleague Yvette Cauchois also analyzed pollucite, this time using their high-resolution X-ray apparatus.^[20] 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 they conducted their work.^[21] 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, however, 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, however, continuously criticized Hulubei's work until she was credited as the sole discoverer of element 87.^[20]

Perey's analysis

Eka-caesium was truly discovered in 1939 by Marguerite Perey of the Curie Institute in Paris, France when she purified a sample of actinium-227 which had been reported to have a decay energy of 220 keV. However, 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, caused by the alpha decay of actinium-227.^[19] 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%.^[24]

Perey named the new isotope actinium-K (now referred to as francium-223)^[19] and in 1946, she proposed the name catium 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.^[19] Perey then suggested francium, after France. This name was officially adopted by the International Union of Pure and Applied Chemistry in 1949,^[4] becoming the second element after gallium to be named after France. It was assigned the symbol Fa, but this abbreviation was revised to the current Fr shortly thereafter.^[25] Francium was the last naturally occurring element to be discovered, following rhenium in 1925.^[19] Further research into francium's structure was carried out by, among others, Sylvain Lieberman and his team at CERN in the 1970s and 1980s.^[26]

Occurrence

Natural

Francium-223 is the result of the alpha decay of actinium-227 and can be found in trace amounts in uranium and thorium minerals.^[3] In a given sample of uranium, there is estimated to be only one francium atom for every 1×10¹⁸ uranium atoms.^[14] It is also calculated that there is at most 30 g of francium in the earth's crust at any time.^[27] This makes it the second rarest element in the crust after astatine.^[4][14]

Synthesized

Francium can be synthesized in the nuclear reaction ¹⁹⁷Au + ¹⁸O → ²¹⁰Fr + 5n. This process, developed by Stony Brook Physics, yields francium isotopes with masses of 209, 210, and 211,^[29] which are then isolated by the magneto-optic trap (MOT).^[28] Other synthesis methods include bombarding radium with neutrons, and bombarding thorium with protons, deuterons, or helium ions.^[24] Francium has not yet, as of 2006, been synthesized in amounts large enough to weigh.^{[3][4][14][30]}

Isotopes

Main article: Isotopes of francium

There are 34 known isotopes of francium ranging in atomic mass from 199 to 232.^[3] Francium has seven metastable nuclear isomers.^[3] Francium-223 and francium-221 are the only isotopes that occur in nature, though the former is far more common.^[31]

Francium-223 is the most stable isotope with a half-life of 21.8 minutes,^[3] and it is highly unlikely that an isotope of francium with a longer half-life will ever be discovered or synthesized.^[24] Francium-223 is the fifth product of the actinium decay series as the daughter isotope of actinium-227.^[16] Francium-223 then decays into radium-223 by beta decay (1149 keV decay energy), with a minor (0.006%) alpha decay path to astatine-219 (5.4 MeV decay energy).^[32]

Francium-221 has a half-life of 4.8 minutes.^[3] It is the ninth product of the neptunium decay series as a daughter isotope of actinium-225.^[16] Francium-221 then decays into astatine-217 by alpha decay (6.457 MeV decay energy).^[3]

The least stable ground state isotope is francium-215, with a half-life of 0.12 μs. (9.54 MeV alpha decay to astatine-211):^[3] Its metastable isomer, francium-215m, is less stable still, with a half-life of only 3.5 ns.^[33]

See also

References