RARE-EARTH METAL

any of a large family of chemical elements consisting of scandium (atomic number 21), yttrium (atomic number 39), and the 15 elements from lanthanum to lutetium (atomic numbers 5771). The rare earths themselves are the oxides of these metals, originally thought to be quite scarce, or mixtures of these oxides, commonly occurring together, such as the first rare earths to be discovered, yttria (1794) and ceria (1803). Not all of the rare-earth metals are as scarce as early estimates indicated. Cerium, the most plentiful, is nearly three times more abundant than lead in the Earth's crust. Thulium, the least plentiful (except for artificially produced promethium), is more abundant than silver, gold, or platinum. Moreover, many of the rare-earth metals are found in meteorites, on the Moon, and in the Sun. The spectra of numerous other stars suggest that they have substantially larger quantities of the rare-earth metals than does the solar system. The rare-earth elements are never found as free metals in the Earth's crust. Pure minerals of the individual elements do not exist in nature; all their minerals consist of mixtures of various rare-earth elements and nonmetals. Monazite and bastnaesite are the principal mineral sources of the rare-earth metals. In terms of physical properties, the rare-earth elements do not resemble one another as closely as was once believed. The melting point of lutetium (1,663 C), for example, differs greatly from that of lanthanum (920 C), and the vapour pressures of ytterbium and europium at 1,000 C are millions of times as great as those of lanthanum and cerium. Nevertheless, the rare-earth metals, particularly those with atomic numbers from 58 to 71 (collectively called the lanthanide series), do share certain common characteristics. Cerium, praseodymium, europium, and neodymium all readily corrode in air. Characteristically, small amounts of nonmetallic impurities tend to intensify such properties (e.g., the light lanthanide metals corrode much more rapidly if calcium or magnesium is present). The rare-earth elements are very similar chemically because of their general similarity in atomic structure. All have three electrons in the outermost shells of their atoms and are therefore trivalent in their compounds. They combine directly with nonmetallic elements, giving rise to extremely stable borides, carbides, oxides, and various other binary compounds.