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(prōmē`thēəm), artificially produced radioactive chemical element; symbol Pm; at. no. 61; mass no. of most stable isotope 145; m.p. 1,042°C;; b.p. 3,000°C; (estimated); sp. gr. unknown; valence +3. Although the chemical and physical properties of promethium are not well defined, it is similar to neodymium and samarium, the rare-earth metalsrare-earth metals,
in chemistry, group of metals including those of the lanthanide series and actinide series and usually yttrium, sometimes scandium and thorium, and rarely zirconium. Promethium, which is not found in nature, is not usually considered a rare-earth metal.
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 preceding and following it in the lanthanide serieslanthanide series,
a series of metallic elements, included in the rare-earth metals, in Group 3 of the periodic table. Members of the series are often called lanthanides, although lanthanum (atomic number 57) is not always considered a member of the series.
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 in Group 3 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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. All its isotopes are radioactive and fairly short-lived. Promethium-145, the most stable isotope, has a half-life of almost 18 years. The most useful isotope is promethium-147 (half-life 2.62 years); it is produced in nuclear reactors. It is a beta emitter and is used in making phosphorescent materials. When it is mixed with a phosphor, the light emitted can be used to power a photocell. It must be used with caution; although the beta rays it emits are relatively harmless, they may produce X rays when they interact with atoms of heavy elements. The existence of promethium was predicted at the beginning of the 20th cent. In 1926, B. S. Hopkins and his coworkers claimed to have discovered the element and proposed the name illinium. About the same time Luigi Rolla and his associates (in Italy) reported its discovery and suggested the name florentium. However, definite chemical identification of the element did not occur until 1947, although it may have been synthesized earlier. J. A. Marinsky, L. E. Glendenin, and C. D. Coryell identified the element by ion-exchange chromatography during the course of experiments at Oak Ridge National Laboratory, Tenn., involving the fission of uranium and subsequent neutron bombardment of neodymium. Since observable quantities of the element have never been found in nature, this identification is considered the first actual discovery of the element. The name promethium was suggested by these investigators and adopted in 1949 by the International Union of Pure and Applied Chemistry.



(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.


Lavrukhina, A. K., and A. A. Pozdniakov. Analiticheskaia khimiia tekhnetsiia, prometiia, astatina i frantsiia. Moscow, 1966.
Trifonov, D. N. Prometii—element no. 61. Moscow, 1968.



A chemical element, symbol Pm, atomic number 61, produced artificially in nuclear reactors; atomic weight of the most abundant separated isotope is 147; a member of the rare-earth group of metals.


a radioactive element of the lanthanide series artificially produced by the fission of uranium. Symbol: Pm; atomic no.: 61; half-life of most stable isotope, 145Pm: 17.7 years; valency: 3; melting pt.: 1042°C; boiling pt.: 2460°C (approx.)