: a radioactive metallic element of the rare-earth group obtained as a fission product of uranium or from neutron-irradiated neodymium see Chemical Elements Table
Example Sentences
Recent Examples on the WebOne promethium isotope, with eighty-eight neutrons, has a half-life of a few days; the other, with eighty-six neutrons, has a half-life of a few years. Neima Jahromi, The New Yorker, 27 Dec. 2019