Ho; at. wt 164.93032; at. no. 67; valence 3. A rare earth metal of the yttrium group; member of the lanthanide series. Naturally occurring isotope (mass number): 165; known artificial radioactive isotopes: 146-164; 166-170. Estimated abundance in earth's crust 1.15-1.4 ppm; occurs in rare earth minerals. Discovered by Soret in 1878 and independently by Cleve in 1879: Cleve, Compt. Rend. 89, 478, 708 (1879). Purification of salts: Holmberg, Z. Anorg. Chem. 71, 226 (1911). Separation from erbium: Driggs, Hopkins, J. Am. Chem. Soc. 47, 363 (1925); from other rare earths: Spedding et al., ibid. 69, 2812 (1947); 76, 2557 (1954). Absorption spectrum: Severin, Z. Phys. 125, 455 (1949). Analysis by means of emission spectra: Smith, Wiggins, Analyst 74, 95 (1949). Toxicity study: Haley et al., Toxicol. Appl. Pharmacol. 8, 37 (1966). 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).
Obtained by passing hydrogen iodide over the anhydr chloride at 600°: Jantsch et al., Z. Anorg. Chem. 207, 353 (1932).