Promethium is the fifth member of the Lanthanide series, being located in the first row below the periodic table of elements. It is a silver colored metal with a melting point of 1042 degrees Celsius. It was identified in 1945 by Jacob Marinsky, Lawrence Glendenin, and Charles Coryell. It is the only radioactive lanthanide with a half-life of 17.7 years. 

Today, Promethium doesn't have many uses outside of research due to its radioactivity. 

Promethium 147 has a half-life of 2.62 years and it undergoes beta decay to form Samarium 147.