Rare-Earth Element

rare-earth element [′rer ‚ərth ′el·ə·mənt]

(chemistry)

The name given to any of the group of chemical elements with atomic numbers 58 to 71; the name is a misnomer since they are neither rare nor earths; examples are cerium, erbium, and gadolinium.

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Rare-Earth Element 

any one of a group of chemical elements belonging to a secondary subgroup of group III of Mendeleev’s periodic system. The elements are scandium (symbol Sc, atomic number [Z] = 21), yttrium (Y, Z = 39), lanthanum (La, Z = 57), and the lanthanides (14 elements wherein Z ranges from 58 to 71). Scandium, however, is not always classed as a rare-earth element.

In the free state, rare-earth elements are metals. The term “rare-earth” was assigned to these elements because they occur relatively rarely in the earth’s crust and because they form refractory oxides that are practically insoluble in water. Prior to and during the early 19th century, such oxides were called common earths. An important characteristic of rare-earth elements is their occurrence together in nature. For example, the mineral monazite, one of the primary sources of these elements, contains phosphates of Y, La, and other rare earths. All rare earths display similar chemical properties; their characteristic oxidation state is +3 (valence of three). In the Sc-Y-La series, the basic properties of oxides and hydroxides become more pronounced from Sc to La. For example, scandium hydroxide Sc(OH)₃ is amphoteric, whereas lanthanum hydroxide La(OH)₃ is a rather strong base.

Actinium (Z = 89) closely resembles the rare-earth elements in chemical properties, but it is generally regarded separately because of its radioactivity (lack of stable isotopes).

S. S. BERDONOSOV