The existence of promethium (for the Greek god, Promethius) was predicted in 1912 when Henry Moseley developed an x-ray method for determining integer atomic numbers of elements. An element was clearly missing between neodymium and samarium. It's existence was not confirmed until 1947 by Marinsky, Glendenin and Coryell.
Historical claims for the discovery of element 61 create an interesting trail from around 1925 in Florence (suggested name: florentium) to America in 1926 (suggested name: illinium). None of the claims, however, could be substantiated and today we know they were not simply a result of fleetingly small samples but rather poor work.
While spectral lines of promethium are evident in the light from some stars, it now seems apparent that no promethium is found in accessible areas of the earth--hence the difficulty in finding any! Initial attempts at synthesis of element 61 in a cyclotron at Ohio State University in 1941 led to the suggested name cyclonium. But the recognized synthesis and identification finally came at Oak Ridge in 1947.
The longest-lived isotope of promethium is Pm-145 with a half-life of 17.7 years. There are no significant commercial uses of the metal and so very little has been produced except for theoretical studies.
Stephen R. Marsden (ChemTopics)