Promethium is a lanthanide series element denoted by the atomic symbol Pm. It has an atomic number of 61 and anatomic weight of 146. Promethium is a silvery-white metal with a melting point of 1,907.6°F (1,042°C). It can be produced by the reduction of promethium (III) fluoride with lithium in a vacuum. Fifteen isotopes of the element are known, and all of them exhibit radioactivity. Promethium-145 has the longest half life, 18 years, but promethium-147 is the best known and most widely used isotope.

In 1913, Henry Gwyn Jeffreys Moseley proposed the concept of atomic numbers, a concept that gave a firm theoretical basis for the periodic law. One consequence of Moseley's proposal was that, for the first time, scientists had a reasonable basis for predicting the number of rare earth chemical elements that should be expected.

Prior to Moseley's work, no one could really predict how many of these...