Hf; at. wt 178.49; at. no. 72; valences 4; also 2, 3. Group IVB (4). Six naturally occurring isotopes: 180 (35.22%); 178 (27.1%); 177 (18.56%); 179 (13.75%); 176 (5.21%); 174 (0.163%; α-emitter, T 2 × 1015 years); artificial isotopes: 157; 158; 168-173; 175; 181-183. Abundance in earth's crust: 5 ppm. Found in all zirconium-contg minerals. Discovered in 1923 by Coster and Hevesy: Nature 111, 79 (1923). Extraction from the mineral cyrtolite: Larsen et al., Inorg. Synth. 3, 67 (1950). Prepd by thermal decompn of its iodide; by reduction of the tetrachloride or of the hydrofluohafniate with metallic sodium; by reduction of the oxide with a mixture of calcium and sodium: van Arkel, de Boer, Z. Anorg. Chem. 148, 345 (1925); de Boer, Fast, ibid. 187, 193 (1930); US 2741628. Spectra: Coster et al., cited by Mellor, A Comprehensive Treatise on Inorganic and Theoretical Chemistry 7, 170 (1930). A dibromide and tribromide have been prepd: Schumb, Morehouse, J. Am. Chem. Soc. 69, 2696 (1947). Reviews of hafnium and its compds: Larsen, “Zirconium and Hafnium Chemistry” in Adv. Inorg. Chem. Radiochem. 13, 1-133 (1970); Bradley, Thornton, “Zirconium and Hafnium” in Comprehensive Inorganic Chemistry vol. 3, J. C. Bailar, Jr. et al., Eds. (Pergamon Press, Oxford, 1973) pp 419-490; R. H. Nielson in Kirk-Othmer Encyclopedia of Chemical Technology vol. 12 (Wiley-Interscience, New York, 3rd ed., 1980) pp 67-80.