Yb; at. wt 173.04; at. no. 70; valences 2, 3. A rare earth metal of the yttrium group; member of the lanthanide series. Naturally occurring isotopes (mass numbers): 168 (0.13%); 170 (3.05%); 171 (14.3%); 172 (21.9%); 173 (16.12%); 174 (31.8%); 176 (12.7%); known artificial radioactive isotopes: 152-167; 169; 175; 177; 178. Estimated abundance in earth's crust: 2.66-3.1 ppm. Occurs in the rare earth minerals xenotime, ytterbite (gadolinite), monazite. Discovered independently: Urbain, Compt. Rend. 145, 759 (1907); and called aldebaranium: von Welsbach, Monatsh. Chem. 29, 181 (1908); 34, 1713 (1913). Sepn and purification: Urbain, Congress of Applied Chemistry [X] 94 (1909); Prandtl, Z. Anorg. Chem. 238, 321 (1938); Spedding et al., J. Am. Chem. Soc. 74, 2783 (1952); 76, 2557 (1954). Spectrum: Exner, Haschek et al., cited by Mellor, A Comprehensive Treatise on Inorganic and Theoretical Chemistry 5, 706 (1929). 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).