Discovered in 1947, Promethium is the only rare-earth that has never been found in nature. This makes this element the least abundant in the earth's crust because there is absolutely none. Promethium belongs to the lanthanide series. It is produced in nuclear reactors by neutron bombardment with neodymium-146 and naturally decay to form promethium-147.

Promethium-147 is used in luminescent coatings for instrument dials and as a power source for semiconductor solar batteries for space use. It is also used as a source of X-rays for medical applications in areas of the world where electrical power is not generally available. Shown above is an oxide of promethium.