2007 Schools Wikipedia Selection. Related subjects: Chemical elements
|Name, Symbol, Number||francium, Fr, 87|
|Chemical series||alkali metals|
|Group, Period, Block||1, 7, s|
|Standard atomic weight||(223) g·mol−1|
|Electron configuration||[Rn] 7s1|
|Electrons per shell||2, 8, 18, 32, 18, 8, 1|
|Density (near r.t.)||? 1.87 g·cm−3|
|Melting point|| ? 300 K
(27 ° C, 80 ° F)
|Boiling point|| ? 950 K
(? 677 ° C, ? 1250 ° F)
|Heat of fusion||ca. 2 kJ·mol−1|
|Heat of vaporization||ca. 65 kJ·mol−1|
|Crystal structure||cubic body centered|
(strongly basic oxide)
|Electronegativity||0.7 (Pauling scale)|
|Ionization energies||1st: 380 kJ/mol|
|Electrical resistivity||? 3 µΩ·m|
|Thermal conductivity||(300 K) ? 15 W·m−1·K−1|
|CAS registry number||7440-73-5|
Francium ( IPA: /ˈfrænsiəm/), also referred to as eka-caesium and actinium K, is a chemical element in the periodic table that has the symbol Fr and atomic number 87. It has the lowest known electronegativity and is the second rarest naturally occurring element. Francium is a highly radioactive alkali metal that decays into astatine, radium, and radon; as an alkali metal, it has one valence electron.
Marguerite Perey discovered francium in 1939. Of the elements added to the periodic table, francium was the last to have been discovered in nature before being synthesized. Outside the laboratory, francium is extremely rare, with trace amounts found in uranium and thorium ores. It exists naturally only as the isotopes francium-223 and francium-221. Francium can be synthesized by several methods, but none has yielded a weighable quantity of the element.
Francium has the highest equivalent weight of any element. It is also the least stable of the first 101 elements: its most stable isotope, francium-223, has a half-life of less than 22 minutes. By contrast, astatine, the second-least stable element, has a maximum half-life of 8.5 hours. All isotopes of francium decay into either astatine, radium, or radon. Francium has the lowest electronegativity of all the known elements at 0.7 on the Pauling scale. Caesium has the second-lowest at 0.79. Liquid francium, if such a substance were created, would have a surface tension of 0.05092 J/ m2 at its melting point.
Francium is an alkali metal whose chemical properties most resemble those of caesium. As an alkali metal, francium has one valence electron. Francium coprecipitates with several caesium salts, such as caesium perchlorate, caesium picrate, and caesium chloroplatinate. This coprecipitation results in small amounts of francium perchlorate, francium picrate, and francium chloroplatinate. Nearly all francium salts are water-soluble.
There are no commercial applications for francium due to its instability and rarity; it has only been used for research purposes, both in the fields of biology and atomic structure. Francium was once thought to be an aid in the diagnosis of cancerous diseases; but, this application has since been deemed impracticable.
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. Studies on the light emitted by laser-trapped francium-210 ions have provided accurate data on transitions between atomic energy levels. These experimental results have been fairly similar to those predicted by quantum theory.
As early as 1870, it was thought by chemists that there should be an alkali metal beyond caesium, with an atomic number of 87. It was then referred to by the provisional name eka-caesium. 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.
Russian chemist D. K. Dobroserdov was the first scientist to claim to have found "eka-caesium". In 1925, he observed weak radioactivity in a sample of potassium, another alkali metal, and concluded that eka-caesium was contaminating the sample. He then published a thesis of his predictions of the properties of eka-caesium, in which he named the element russium after his home country. Shortly thereafter, Dobroserdov began to focus on his teaching career at the Polytechnic Institute of Odessa, and he did not pursue the element further.
The following year, English chemists Gerald J. F. Druce and Frederick H. Loring analyzed X-ray photographs of manganese(II) sulfate. 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.
In 1930, Professor 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. In 1934, however, Professor MacPherson of UC Berkeley disproved the effectiveness of Allison's device and the validity of this false discovery.
In 1936, Romanian chemist Horia Hulubei and his French colleague Yvette Cauchois also analyzed pollucite, this time using their high-resolution X-ray apparatus. They observed several weak emission lines which they presumed to be 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. 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.
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 thorium, radium, lead, bismuth, or thallium being the unknown element. The new product exhibited chemical properties of an alkali metal (such as co-precipitating with caesium salts), which led Perey to believe that it was element 87, caused by the alpha decay of actinium-227. Perey then attempted to determine the proportion of beta decay to alpha decay in actinium-227. Her first test put the alpha branching at .6%, which she later revised to 1%.
Perey named the new isotope actinium-K, which we now refer to as francium-223, 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. Perey then suggested francium as an homage to the country in which she discovered it. This name was officially adopted by the International Union of Chemists in 1949. and assigned the symbol Fa; but, this abbreviation was revised to the current Fr shortly thereafter. Francium was the last naturally occurring element to be discovered, following rhenium in 1925. Further research into francium's structure was carried out by Sylvain Lieberman and his team at CERN in the 1970s and 1980s, among others.
Francium-223 is the result of the alpha decay of actinium-227 and can be found in trace amounts in uranium and thorium minerals. In a given sample of uranium, there is estimated to be only one francium atom for every 1×1018 uranium atoms. It is also calculated that there is at most 30 g of francium in the earth's crust at any time. This makes it the second rarest element in the crust after astatine.
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, which are then isolated by the Magneto Optic Trap (MOT). Other synthesis methods include bombarding radium with neutrons, and bombarding thorium with protons, deuterons, or helium ions. No weighable amount of francium has been synthesized.
There are 34 known isotopes of francium ranging in atomic mass from 199 to 232. Francium has seven metastable nuclear isomers. Francium-223 and francium-221 are the only isotopes that occur in nature, though the former is far more common.
Francium-223 is the most stable isotope with a half-life of 21.8 minutes, and it is highly unlikely that an isotope of francium will ever be discovered or synthesized with one longer. Francium-223 is the fifth product of the actinium decay series as the daughter isotope of actinium-227. 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).
Francium-221 has a half-life of 4.8 minutes. It is the ninth product of the neptunium decay series as a daughter isotope of actinium-225. Francium-221 then decays into astatine-217 by alpha decay (6.457 MeV decay energy).
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): Its metastable isomer, francium-215m, is less stable still, with a half-life of only 3.5 ns.