scandium (skănˈdēəm) [key], metallic chemical element; symbol Sc; at. no. 21; at. wt. 44.95591; m.p. 1,541°C; b.p. 2,831°C; sp. gr. 2.99 at 20°C; valence +3. Scandium is a soft silver-white metal. It is a member of Group 3 of the periodic table; because of its chemical and physical properties, its scarcity, and the difficulty in extracting the metal, it is sometimes regarded as one of the rare-earth metals. At ordinary temperatures it crystallizes in a hexagonal close-packed structure. It tarnishes slightly when exposed to air. It reacts with many acids. It forms an oxide and a number of colorless salts. Its compounds are found widely distributed in minute amounts in nature. It is a major component of the rare Norwegian mineral thortveitite. It is found in many of the rare-earth minerals and in certain tungsten and uranium ores. Scandium is found in relatively greater abundance in the sun and certain stars than on earth. The metal has little commercial importance. In 1970 pure scandium cost several thousand dollars per pound. Scandium oxide (scandia) finds use as a catalyst and in making crucibles and other ceramic parts. Scandium sulfate in very dilute aqueous solution is used in agriculture as a seed treatment to improve the germination of corn, peas, wheat, and other plants. Scandium was discovered by L. F. Nilson in 1879 by spectroscopic analysis of euxenite and gadolinite. It was later shown by P. T. Cleve to correspond to the ekaboron predicted in 1871 by Mendeleev from his periodic law.