francium fran·ci·um (frān'sē-əm)
n.
Symbol Fr
An extremely unstable radioactive element of the alkali metals, having [approx] 19 isotopes, the most stable of which is Fr 223 with a half-life of 21 minutes. Atomic number 87; valence 1.
| francium (frān'sē-əm) Symbol Fr An extremely unstable, radioactive element of the alkali group. It is the heaviest metal of the group. Francium occurs in nature, but less than 28.35 g (1 oz) is present in the Earth's crust at any time. It has approximately 19 isotopes, the most stable of which is Fr 223 with a half-life of 21 minutes. Atomic number 87; valence 1. See Periodic Table. |
heaviest chemical element of Group 1 (also called Group Ia) in the periodic table, the alkali metal group. It exists only in short-lived radioactive forms. Natural francium cannot be isolated in visible, weighable amounts, for only about 30 grams (about 1 ounce) occur at any time in the entire crust of Earth. Marguerite Perey discovered francium (1939) while studying actinium-227, which decays by negative beta decay (electron emission) to an isotope of thorium (thorium-227) and by alpha emission (about 1 percent) into an isotope of francium (francium-223) that was formerly called actinium K (AcK) and is a member of the actinium decay series. Though it is the longest-lived isotope of francium, francium-223 has a half-life of only 21 minutes. Isotopes of francium with masses between 204 and 224 have been artificially prepared, and, because natural francium cannot be concentrated, it is also prepared by neutron irradiation of radium to produce actinium, which decays to produce traces of francium. The chemistry of francium can be studied only by methods designed for trace quantities. In all respects, its observed behaviour, including the oxidation state of +1, is that to be expected of an alkali element filling a place just below cesium in the periodic table of the elements