PROMETHIUM

(Pm), chemical element, only rare-earth metal of transition Group IIIb of the periodic table not detected in nature. Conclusive chemical proof of the existence of promethium, the last of the rare-earth elements to be discovered, was obtained (1947) by J.A. Marinsky, L.E. Glendenin, and C.D. Coryell, who isolated the radioactive isotope promethium-147 (2.7-year half-life) from uranium fission products at the research site at Oak Ridge, Tenn. Identification was firmly established by spectroscopy. Earlier investigators thought that they had found the element with atomic number 61 in naturally occurring rare earths and had prematurely called it illinium and florentium. Promethium-147 is effectively separated from the other rare-earth fission products by an ion-exchange method. Its soft beta radiation is converted to electricity in miniature batteries formed by sandwiching promethium between wafers of a semiconductor such as silicon; these batteries operate in extreme temperatures for five years. Promethium has also been prepared by slow neutron bombardment of the isotope neodymium-146; the resultant isotope, neodymium-147, decays by electron emission to promethium-147. The metal itself was first prepared (1963) by reduction of the fluoride, PmF3, with lithium. All of the isotopes of promethium are unstable; the longest lived is promethium-145 (18-year half-life). Because of the short half-lives of its isotopes, any promethium that might result from spontaneous fission of uranium in uranium ores would occur in infinitesimal concentrations. The physical and chemical properties of promethium are those of a typical rare earth. It is trivalent in its compounds and solutions, most of which are pink or rose. atomic number 61 stablest isotope (145) melting point 1,080 C boiling point 2,460 C specific gravity valence 3 electronic config. 2-8-18-23-8-2 or (Xe)4f 55d06s2