Promethium (
/prɵˈmiːθiəm/ pro-mee-thee-əm), originally prometheum, is
a chemical element with the symbol Pm and atomic number 61. It is notable for being the only element besides technetium all of whose isotopes are radioactive but which is followed in the periodic table by chemical elements with stable isotopes. Chemically, promethium is a lanthanide, which forms salts when combined with other elements. Promethium shows only one stable oxidation state of +3; however, a few +2 compounds may exist.
There are three possible sources for promethium: rare decays of natural (primordial) neodymium (producing promethium-150), europium (promethium-147), and uranium (various isotopes). The most stable isotope, promethium-145, has a very low rate of alpha decay, so that the alpha half-life is long enough for the presence of primordial promethium to be theoretically possible; however, this has not been experimentally confirmed. Practical applications exist only for chemical compounds of promethium-147, which are used in luminous paint, atomic batteries and thickness measurement devices. Since natural promethium is exceedingly scarce, the element is typically synthesized by bombarding uranium-235 (enriched uranium) with thermal neutrons (for promethium-147).