Tb; at. wt 158.92535; at. no. 65; valences 3, 4. A lanthanide; belongs to the yttrium group of rare earth metals. Naturally occurring isotope (mass number): 159; known artificial radioactive isotopes: 144-158; 160-164. Abundance in earth's crust: 0.91-1.2 ppm; occurs in small quantities in monazite, cerite, gadolinite and other rare earth minerals. Discovered by Mosander, Skand. Naturf"or. F"orh. 3, 387 (1842); Philos. Mag. [3] 23, 241 (1843). Sepn by fractional crystn and precipitation: Urbain, Compt. Rend. 139, 736 (1904); 141, 521 (1905); James, Bissel, J. Am. Chem. Soc. 36, 2060 (1914); by ion exchange: Spedding et al., ibid. 76, 2557 (1954). Prepn of metal by electrodeposition: eidem, J. Electrochem. Soc. 100, 442 (1953). Absorption spectrum: Urbain, loc. cit. Toxicity study: Haley, J. Pharm. Sci. 54, 663 (1965). Reviews of prepn, properties and compds: The Rare Earths, F. H. Spedding, A. H. Daane, Eds. (Krieger, Huntington, N.Y., 1971, reprint of 1961 ed.) 641 pp; Hulet, Bode, “Separation Chemistry of the Lanthanides and Transplutonium Actinides” in MTP Int. Rev. Sci.: Inorg. Chem., Ser. One vol. 7, K. W. Bagnall, Ed. (University Park Press, Baltimore, 1972) pp 1-45; Moeller, “The Lanthanides” in Comprehensive Inorganic Chemistry vol. 4, J. C. Bailar, Jr. et al., Eds. (Pergamon Press, Oxford, 1973) pp 1-101; F. H. Spedding in Kirk-Othmer Encyclopedia of Chemical Technology vol. 19 (John Wiley & Sons, New York, 3rd ed., 1982) pp 833-854; Chemistry of the Elements, N. N. Greenwood, A. Earnshaw, Eds. (Pergamon Press, New York, 1984) pp 1423-1449. Brief review of properties: G. T. Seaborg, Radiochim. Acta 61, 115-122 (1993).