RARE-EARTH ELEMENT

any of a large family of chemical elements consisting of scandium (atomic number 21), yttrium (atomic number 39), and 15 elements from lanthanum to lutetium (atomic numbers 5771). They form a series of 17 chemically similar metals, all but one of which occur in nature. Often they are called simply rare earths, but this is a misnomer because the term earth properly is applied to the oxide of a metal rather than to the element itself. The rare-earth elements are not even particularly rare, though for a long time they were thought to be. The 17 rare-earth elements are: scandium (Sc), yttrium (Y), lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu). Until the mid-20th century, there was not much use for pure rare-earth elements or compounds except cerium and lanthanum; mixtures of the rare earths, however, had found metallurgical and other uses. By the 1970s three of these elements, yttrium, gadolinium, and europium, were being used in the red phosphors for colour television. Figure 1: Modern version of the periodic table of the elements. To see more information about an In the periodic table of the elements (see Figure), the rare-earth elements comprise three members of Group IIIb and all 14 members of one of two series of elements generally written apart from the main table. This long series is known collectively as the lanthanide series because it directly follows lanthanum in a different form of the table. The rare-earth elements all have certain common features in the electronic structure of their atoms, which is the fundamental reason for their chemical similarity. The aqueous chemistry of all the rare earths is very similar and changes only slightly in progressing along the lanthanide series. Because of this similarity, it is difficult to separate individual rare earths. In the few cases in which the rare-earth ion can be oxidized or reduced to another valency, however, chemical separations can be carried out readily. Also, artificial mixtures of elements far apart in the series can be separated easily. All of these elements form trivalent compounds, and in the crystal lattices (the regular arrangement of atoms in the solid forms) of such compounds, one rare-earth ion readily replaces another. The rare-earth metals when heated react strongly with nonmetallic elements to form very stable compounds. They are never found as the free metals in the Earth's crust. Pure minerals of individual rare earths do not exist in nature; all their minerals contain mixtures of the rare-earth elements. Promethium is never found in the Earth's crust since it has no stable isotopes and is produced only by nuclear reactions; it can, however, be obtained in quantity from the fission products formed in nuclear reactors. The chemical properties of scandium differ sufficiently from those of other rare-earth elements for it to have become segregated from them by the action of geological processes. Scandium seldom is associated with the rare earths in minerals. Additional reading F.H. Spedding and A.H. Daane (eds.), The Rare Earths (1961), broadly reviews rare-earth chemistry and metallurgy to 1960. Karl A. Gschneider, Jr., Rare Earth Alloys (1961), also provides a review to 1960. C. Michael Lederer and Virginia S. Shirley (eds.), Table of Isotopes, 7th ed. (1978), includes a review of radioactivity of the rare-earth elements. Specific aspects are treated in detail in R.J. Elliott (ed.), Magnetic Properties of Rare Earth Metals (1972); P. Henderson (ed.), Rare Earth Element Geochemistry (1984); Bruce R. Lipin and G.A. McKay (eds.), Geochemistry and Mineralogy of Rare Earth Elements (1989); E.J. Wheelwright (ed.), Promethium Technology (1973); Norman M. Edelstein (ed.), Lanthanide and Actinide Chemistry and Spectroscopy (1980); Karl A. Gschneider, Jr., and LeRoy Eyring (eds.), Handbook on the Physics and Chemistry of Rare Earths (1978); Karl A. Gschneider, Jr. (ed.), Industrial Applications of Rare Earth Elements (1981); and C.K. Gupta and T.S. Krishnan, Rare Earths: Applications and Technology (1988). Frank Harold Spedding The Editors of the Encyclopdia Britannica