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(Pm), a radioactive chemical element of group III of Mendeleev’s periodic table; one of the lanthanides. Atomic number, 61.
There are 16 known promethium isotopes, with mass numbers 141–154, and two nuclear isomers. The most stable isotope is the almost unobtainable 145Pm (half-life T½ ~ 18 years during radioactive decay by electron capture), and β-radioactive l47Pm (T½ = 2.7 years) is the isotope with the greatest importance.
The search for promethium, begun in the early 20th century, for a long time proved futile. Attempts to synthesize element 61 with the aid of nuclear reactions were initiated in 1938 by the American physicists M. Pool and L. Quill, who irradiated neo-dymium (atomic number 60) with deuterons; however, the scientists were unable to prove the formation of a new element. The discovery date for promethium is considered to be 1945, when the American researchers J. Marinsky, L. Glendenin, and C. Coryell isolated the element from uranium fission products using ion-exchange chromatography and identified it chemically. The new element was named after Prometheus. In 1968 the existence of promethium—isotope 147Pm—was discovered in uranium-bearing ores. This isotope, in the quantity ~4 × 10-15g per kg ore, is formed by the nuclear fission of 238U and 235U.
Promethium is a metal with a density of 7.26 g/cm3 and a melting point of 1080°C; the electronic configuration of the highest energy levels is 4f56s2. The element has the chemical properties typical of lanthanides and exhibits an oxidation number, or valence number, of + 3 in compounds. The light-brown hydroxide Pm(OH)3 yields the oxide Pm2O3 upon calcination. Water-soluble promethium salts include the yellow chloride PmCl3 and pink nitrate Pm(NO3)3; examples of insoluble salts are the fluoride PmF3 and oxalate Pm2(C2O4)3. 10H2O.
Promethium, composed chiefly of the isotope 147Pm, is obtained from 235U fission fragments that are formed during the operation of nuclear reactors; several hundred grams of promethium accumulate in large reactors over a one-year period. Promethium is isolated from the other lanthanides in fission fragments by chromatography and is used in the preparation of long-lived phosphors. The β-rays of l47Pm make it possible to measure the thickness of various materials, hence the use of promethium in thickness gauges employing radioisotopes. The isotope l47Pm also serves as a base for miniature, nuclear-powered batteries with a service life of several years. In these batteries, the energy of 147Pm β-rays is converted into electric energy.
REFERENCESLavrukhina, A. K., and A. A. Pozdniakov. Analiticheskaia khimiia tekhnetsiia, prometiia, astatina i frantsiia. Moscow, 1966.
Trifonov, D. N. Prometii—element no. 61. Moscow, 1968.
S. S. BERDONOSOV