Most elements of the periodic table, and in particular the metallic elements, are divided into groups based primarily on such chemical properties as reactivity, oxidation number, or the formulae of binary compounds. Between 1751, when the Swedish mineralogist A. F. Cronstedt discovered a heavy mineral that was later found to contain the element cerium, and about 1920, the designation rare earth was given to any unfamiliar, but naturally occurring metal oxide. However, the name used to describe this group of elements is now both inappropriate and a source of confusion: inappropriate because the elements are no longer rare, with the exception of promethium (¹⁴⁷Pm, Z=61) which has no stable isotopes. Part of the confusion lies in determining which elements should belong to the group. The number of elements that are included in the rare earth group varies from 14 to 32 depending on the literature citation. It is...