Title: Thorium
CAS Registry Number: 7440-29-1
Literature References: Th; at. wt 232.03806 (characteristic terrestrial isotopic composition); at. no. 90; valence 4. No stable nuclides. Naturally occurring isotopes (mass numbers): 232 (~100%, T½ 1.40 ´ 1010 years, longest-lived known isotope, a-decay, rel. at. mass 232.0380); 228 (natural decay product of 232Th); 227, 231 (natural decay products of 235U); 230 (T½ 7.54´104 years, rel. at. mass 230.0331), 234 (natural decay products of 238U); other isotopes: 213-226, 229, 233, 235, 236. 232Th found in nature undergoes natural decay by the emission of a-, b-, and g-rays, eventually forming 208Pb. Occurs in the minerals thorite, thorianite, orangite, yttrocrasite; in monazite sand (principal ore of commercial significance); present in the earth's crust 8-15 ppm. Isoln from thorite (ThSiO4): J. J. Berzelius, K. Sven. Ventenskapsakad. Handl. 9, 1 (1829); Pogg. Ann. 16, 385 (1829). Prepn of metal: idem, Ann. Phys. 16, 385 (1829); Marden, Trans. Electrochem. Soc. 66, 39 (1934). Monographs: F. L. Cuthbert, Thorium Production Technology (Addison-Wesley, Reading, Mass., 1958); L. Grainger, Uranium and Thorium (Pitman, London, 1958). Reviews: Mellor's vol. 7, 174-253 (1930); "The Actinides," in Comprehensive Inorganic Chemistry vol. 5, J. C. Bailar, Jr., et al., Eds. (Pergamon Press, Oxford, 1973) pp 1-715; L. I. Katzin in Kirk-Othmer Encyclopedia of Chemical Technology vol. 22 (Wiley-Interscience, New York, 3rd ed., 1983) pp 989-1002; L. I. Katzin, D. C. Sonnenberger in The Chemistry of the Actinide Elements vol. 1, J. J. Katz et al., Eds. (Chapman and Hall, New York, 1986) pp 41-101. Review of prepn and purification: J. C Spirlet et al., Adv. Inorg. Chem. 31, 1-40 (1987). Review of toxicology and health effects: Toxicological Profile for Thorium (PB91-180448, 1990) 174 pp; Patty's Industrial Hygiene and Toxicology vol. 2C, G. D. Clayton, F. E. Clayton, Eds. (Wiley-Interscience, New York, 4th ed., 1994) pp 2249-2258.
Properties: Grayish-white, lustrous, radioactive metal; somewhat ductile and malleable. Two alloptropic phases: face-centered cubic a-form, d25 11.724, transforms to b-form at 1360°; body-centered cubic b-form transforms to liquid at mp 1750° (Katzin, Sonnenberger). Also reported as mp 1690° (Cuthbert). bp ~3800°. Heat of fusion <19.2 kJ/mol. Heat of vaporization ~586 kJ/mol. Heat capacity (25°) 27.32 J/mol.K. Darkens on prolonged exposure to air. Finely divided metal is pyrophoric in air. HCl attacks metal vigorously, leaving up to 25% as an undissolved residue. Nitric acid passivates metal. Dilute HF and H2SO4 and concentrated H3PO4 and HClO4 attack thorium slowly, with evolution of H. Metal not attacked by alkali hydroxides.
Melting point: mp 1750° (Katzin, Sonnenberger); mp 1690° (Cuthbert)
Boiling point: bp ~3800°
Density: d25 11.724
Use: As fuel in nuclear reactors, as source of fissionable 233U. In manuf incandescent gas-light mantles, welding electrodes, ceramics. As hardener in Mg alloys; for filament coatings in incandescent lamps and vacuum tubes; as chemical catalyst.

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