tating with caesium salts), which led Perey to believe that it was element 87, caused by the alpha decay of actinium-227. [21] 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%. [26]
Perey named the new isotope actinium-K (now referred to as francium-223) [21] 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. IrГЁne Joliot-Curie, one of Perey's supervisors, opposed the name due to its connotation of cat rather than cation. [21] 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. [27] Francium was the last element discovered in nature, rather than synthesized, following rhenium in 1925. [21] Further research into francium's structure was carried out by, among others, Sylvain Lieberman and his team at CERN in the 1970s and 1980s. [28]
5 . Occurrence
5.1 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 Г— 1018 uranium atoms. [15] It is also calculated that there is at most 30 g of francium in the earth's crust at any time. [29] This makes it the second rarest element in the crust after astatine. [4] [15]
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Figure. 0. This sample of uraninite contains about 100,000 atoms (3.3? 10? 20 g) of 223Fr at any given time. [15]
5.2 Synthesized
Francium can be synthesized in the nuclear reaction 197Au + 18O в†’ 210Fr + 5n
This process, developed by Stony Brook Physics, yields francium isotopes with masses of 209, 210, and 211, [31] which are then isolated by the magneto-optical trap (MOT). [30] The production rate of a particular isotope depends on the energy of the oxygen beam. An 18O beam from the Stony Brook LINAC creates 210Fr in the gold target with the nuclear reaction 197Au + 18O = 210Fr + 5n. The production required some time to develop and understand. It was critical to operate the gold target very close to its melting point and to make sure that its surface was very clean. The nuclear reaction imbeds the francium atoms deep in the gold target, and they must be removed efficiently. The atoms diffuse fast to the surface of the gold target and are released as ions. The francium ions are guided by electrostatic lenses until they land into a surface of hot yttrium and become neutral again. The francium is then injected into a glass bulb. A magnetic field and laser beams cool and confine the atoms. Although the atoms remain in the trap for only about 20 seconds before escaping (or decaying), a steady stream of fresh atoms replaces those lost, keeping the number of trapped atoms roughly constant for minutes or longer. Initially, about 1000 francium atoms were trapped in the experiment. This was gradually improved and is capable of trapping over 300,000 neutral atoms of francium a time. Although these are neutral "metallic" atoms ("francium metal"), they in a gaseous unconsolidated state. Enough francium is trapped that a video camera can capture the light given off by the atoms as they fluoresce. The atoms appear as a glowing sphere about 1 millimeter in diameter. This was the very first time that anyone had ever seen francium. The researchers can now make extremely sensitive measurements of the light emitted and absorbed by the trapped atoms, providing 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. Other synthesis methods include bombarding radium with neutrons, and bombarding thorium with protons, deuterons, or helium ions. [26] Francium has not yet, as of 2009 [update], been synthesized in amounts large enough to weigh. [3] [4] [15] [32]
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Figure. 1. Neutral francium atoms can be trapped in the MOT using a magnetic field and laser beams. [30]
6 . Isotopes
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. [33]
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-l...
