promethium (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 metals preceding and following it in the lanthanide series in Group 3 of the periodic table. 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.