Promethium is a chemical element with the symbol Pm and atomic number 61. It is part of the lanthanide series of the periodic table. Promethium is one of the rarest elements on Earth and is radioactive, with no stable isotopes. Its most common isotopes are promethium-145 and promethium-147. Promethium does not occur naturally in significant quantities; it was discovered in 1945 by chemists Glenn T.
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