Title: Protactinium
CAS Registry Number: 7440-13-3
Additional Names: Protoactinium
Literature References: Pa; at. no. 91; at. wt 231.03588 (characteristic naturally occurring isotopic mixture); valences 3, 4, 5. No stable nuclides; known isotopes (mass numbers): 216, 222-238; naturally occurring isotopes: 231, 234m, 234. First isotope discovered, 234mPa (T½ 1.17 minutes); called brevium, uranium X2 or UX2, natural decay product of 238U. Modes of decay: g by isomeric transition to yield 234Pa; b- to yield 234U. Longest-lived isotope, 231Pa (T½ 3.276 ´ 104 years, rel. at. mass 231.0359); natural decay product of 235U. Decays by a emission; parent of 227Ac. 234Pa (T½ 6.75 hrs); called Uranium Z or UZ; natural decay product of 238U. Decays by b- emission. Discovery of m234Pa: K. Fajans, O. H. Göhring, Naturwissenschaften 1, 339 (1913), C.A. 7, 3916 (1913); eidem, Phys. Z. 14, 877 (1913), C.A. 7, 297 (1913); of 231Pa: O. Hahn, L. Meitner, ibid. 19, 208 (1918); and independently by F. Soddy, J. A. Cranston, Proc. Roy. Soc. 94A, 384 (1918); of 234Pa: O. Hahn, Ber. 54B, 1131 (1921). Prepn of metal: A. V. Grosse, M. Agrass, J. Am. Chem. Soc. 56, 2200 (1934). Review of discovery: K. Fajans, D. F. C. Morris, Nature 244, 137-138 (1973); of use in nuclear fuel cycle: O. L. Keller, Radiochim. Acta 25, 211-223 (1978). Reviews: Comprehensive Inorganic Chemistry vol. 5, J. C. Bailar, Jr. et al., Eds. (Pergamon Press, Oxford, 1973) passim; H. W. Kirby in The Chemistry of the Actinide Elements vol. 1, J. J. Katz et al., Eds. (Chapman and Hall, New York, 1986) pp 102-168.
Properties: Shiny, silvery, malleable, ductile metal. Body-centered tetragonal crystal structure. Easily tarnished in air to an undetermined oxide. mp 1560°; also reported as 1575° (Bailar). bp 4227°. d25 15.37. Reacts with H2 at 250-300° to form PaH3. In dil solns of HF, is deposited (10-96%) on Be, Al, Mn, Zn, and Pl; gives small deposits on Cr, Ta, Fe, Cd, Ni, Cu, Hg, W: Camarcat et al., J. Chim. Phys. 46, 153-157 (1949).
Melting point: mp 1560°
Boiling point: bp 4227°
Density: d25 15.37
CAUTION: Radiation hazard; handling requires special equipment and shielding facilities (Katz et al., loc. cit. vol 2, p 1128). Inhalation hazard in insol form; general hazard if absorbed systemically. Max permissible concn of insoluble form in air: 4 ´ 10-11 mCi/cc; of sol form in air: 4 ´ 10-13 mCi/cc; Natl. Bur. Stand. Handb. 69, 83 (1959).
Use: 233Pa as intermediate in production of fissile 233U in thorium breeder reactors.

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