Any of a large class of chemical element s including scandium (atomic number 21), yttrium (39), and the 15 elements from 57 (lanthanum) to 71 (see lanthanide s).
The rare earths themselves are pure or mixed oxide s of these metal s, originally thought to be quite scarce; however, cerium , the most plentiful, is three times as abundant as lead in the Earth's crust. The metals never occur free, and the pure oxides never occur in minerals. These metals are similar chemically because their atomic structures are generally similar; all form compounds in which they have valence 3, including stable oxides, carbide s, and borides.