Fr, a radioactive chemical element in Group I of Mendeleev’s periodic system; an alkali metal. Atomic number, 87. Francium has no stable isotopes. More than 20 isotopes are known, with mass numbers from 203 to 229, but they are all highly unstable; the most long-lived isotope is the β-emitting 223 Fr(T½ = 21.8 min), which occurs in nature.
The existence and principal properties of the heaviest analogue of the alkali metals were predicted by D. I. Mendeleev in 1870, but attempts to discover the element in nature were long unsuccessful. It was only in 1939 that the Frenchwoman M. Perey succeeded in proving that 227 Ac nuclei, in 12 out of 1,000 cases, emit an α-particle and thereby transform into nuclei of element with atomic number 87 and a mass number of 223, which Perey also isolated. She named the new element in honor of her native country. As a member of the radioactive series 235U, 223Fr occurs in nature in negligible quantities: 1 atom of Fr per 3 × 1018 atoms of natural uranium. According to calculations, the earth’s surface layer of 1.6 km thickness contains approximately 24.5 g of Fr.
Owing to the rapidity of radioactive decay, the properties of francium are studied only using samples containing negligibly small quantities of the element.
The configuration of the outer electron shell of a francium atom is 7s1. The atomic radius is 2.77 angstroms, and the ionic radius of Fr+ is about 1.81 angstroms. Estimates show that the melting point of metallic francium is 8.0°C, the boiling point is 620°C, and the density is 2.48 g/cm3; the ionization potential of Fr—Fr+ is 3.98 eV. Francium exhibits an oxidation state +1 in all compounds. In solutions it behaves like a typical alkali metal and is most similar to cesium with respect to properties. Almost all the francium salts are readily soluble in water; during crystallization, francium is isomorphically precipitated with cesium perchlorate, cesium hexachloroplatinate, and other cesium salts.
Francium can be separated from other natural radioactive elements, such as Ac and Th, by extraction or by chromatographic methods. The isotope 223Fr has limited applications; it is used for determining 227Ac by the β-emission of 223Fr, which is a daughter isotope of 227Ac, and for studying the migration of heavy alkali-metal ions in biological objects.
S. S. BERDONOSOV