Spedding, Frank H. Formerly, Ames Laboratory, Energy Research and Development Administration, Iowa State University, Ames, Iowa.
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The group of 17 chemical elements with atomic numbers 21, 39, and 57–71; the name lanthanides is reserved for the elements 58–71. The name rare earths is a misnomer, because they are neither rare nor earths. The early Greeks believed that everything in the world was made of four elements: air, earth, fire, and water. The earths were substances which could not be changed with heat by the temperatures then available to the scientist; and in the early part of the nineteenth century, when the first rare earths were discovered, they resembled the common earths, which were really oxides of magnesium, calcium, and aluminum. Since the rare earths were found in very rare minerals, they were thus called rare earths. They are not rare, however, since cerium is reported to be more abundant in the Earth's crust than tin; yttrium more abundant than lead; and even the scarce rare earths, except promethium, more abundant than the platinum-group elements. All these elements form trivalent bonds, and when their salts are dissolved in water, they ionize to form trivalent ions and the solutions exhibit very similar chemical properties. The elements scandium, yttrium, lanthanum, and actinium in the II column of the extended periodic table show similar properties in aqueous solution. Yttrium and lanthanum are always found associated with the rare earths in nature.
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