pro·me·thi·um   (prə-mē'thē-əm)    n.   Symbol Pm A radioactive rare-earth element prepared by fission of uranium or by neutron bombardment of neodymium, having 14 isotopes with mass numbers ranging from 141 to 154 and used as a source of beta rays. Atomic number 61; melting point 1,168°C; boiling point 2,460°C; valence 3. See Table at element. [From Prometheus.]

Main Entry: pro·me·thi·um
Pronunciation: pr&-'mE-thE-&m
Function: noun
: a radioactive metallic element of the rare-earth groupobtained as a fission product of uranium or from neutron-irradiated neodymium —symbol Pm; —see ELEMENT table

promethium pro·me·thi·um (prə-mē'thē-əm)
n.
Symbol Pm
A radioactive rare-earth element prepared by fission of uranium. Pm 145 is the longest-lived isotope with a half-life of 17.7 years. Atomic number 61; melting point 1,042°C; boiling point 3,000°C; valence 3.

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

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