Rare Earth Elements Defined:
The term “rare earth” is generally used to refer to a group of 17 metals that are used in a wide variety of high technology applications. The International Union of Pure and Applied Chemistry defines “rare earth metals” as all of the lanthanoids (atomic numbers 57 – 71) in addition to Scandium and Yttrium (atomic numbers 21 and 39). Promethium (61) is a member of this group, but does not have a stable isotope and therefore does not occur naturally.
Periodic Table of the Elements