9374. Thorium

Nomenclature

CAS number: 7440-29-1

Description and 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%, T 1.40 × 1010 years, longest-lived known isotope, α-decay, rel. at. mass 232.0380); 228 (natural decay product of 232Th); 227, 231 (natural decay products of 235U); 230 (T 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 α-, β-, and γ-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 α-form, d25 11.724, transforms to β-form at 1360°; body-centered cubic β-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.

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.