Lu; at. wt 174.967; at. no. 71; valence 3. A lanthanide; belongs to the yttrium group of rare earth metals. Naturally occurring isotopes (mass numbers): 175 (97.41%); 176 (2.59%), radioactive, T 3.59 × 1010 years, β emitter. Known artificial radioactive isotopes: 151; 153-174; 177-181. Abundance in earth's crust: 0.75-0.8 ppm. Occurs in xenotime, gadolinite and other rare earth minerals. Discovered independently: Urbain, Compt. Rend. 145, 759 (1907); 146, 406 (1908); 152, 141 (1911); and called cassiopeium: von Welsbach, Sitzungsber. Akad. Wiss. Wien 1907, 468; idem, Monatsh. Chem. 29, 181 (1908); 30, 695 (1909). Isoln in form of salt: Prandtl, Z. Anorg. Chem. 238, 321 (1938); Marsh, J. Chem. Soc. 1952, 4804. Sepn by ion exchange: Spedding et al., J. Am. Chem. Soc. 76, 2557 (1954). 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).