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(frăn`sēəm) [from France], radioactive chemical element; symbol Fr; at. no. 87; mass no. of most stable isotope 223; m.p. about 27°C; (estimated); b.p. 677°C; (estimated); sp. gr. unknown; valence +1. Francium is extremely rare; its most stable isotope (half-life about 22 minutes) occurs naturally, to a very limited extent, in uranium minerals. More than 30 other isotopes of francium are known; some are prepared by bombarding thorium with protons, deuterons, or alpha particles.

Francium is one of the alkali metalsalkali metals,
metals found in Group 1 of the periodic table. Compared to other metals they are soft and have low melting points and densities. Alkali metals are powerful reducing agents and form univalent compounds.
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 found in Group 1 of the periodic tableperiodic table,
chart of the elements arranged according to the periodic law discovered by Dmitri I. Mendeleev and revised by Henry G. J. Moseley. In the periodic table the elements are arranged in columns and rows according to increasing atomic number (see the table entitled
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. Because it is so rare, its chemical and physical properties are not known, but it is believed to resemble cesium. The element was discovered in 1939 by Marguerite Perey at the Curie Institute in Paris as a product of the radioactive disintegration of actinium. In the United States it was at one time called virginium.



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.


Hyde, E. Radiokhimiia frantsiia i thoriia. Moscow, 1961. (Translated from English.)
Lavrukhina, A. K., and A. A. Pozdniakov. Analiticheskaia khimiia tekhnetsiia, prometiia, astatina i frantsiia. Moscow, 1966.



A radioactive alkali-metal element, symbol Fr, atomic number 87, atomic weight distinguished by nuclear instability; exists in short-lived radioactive forms, the chief isotope being francium-223.


an unstable radioactive element of the alkali-metal group, occurring in minute amounts in uranium ores. Symbol: Fr; atomic no.: 87; half-life of most stable isotope, 223Fr: 22 minutes; valency: 1; melting pt.: 27?C; boiling pt.: 677?C