Title: Neptunium
CAS Registry Number: 7439-99-8
Literature References: Np; at. no. 93; valence 3, 4, 5, 6, 7. First man-made transuranium element; no stable nuclides. Known isotopes (mass numbers): 227-242. Discovery of isotope 239 (T½ 2.355 days, b-decay, rel. at. mass 239.0529): E. McMillan, P. Abelson, Phys. Rev. 57, 1185 (1940); of isotope 237 (a-emitter, T½ 2.14 ´ 106 years, longest-lived known isotope, rel. at. mass 237.0482): A. C. Wahl, G. T. Seaborg, ibid. 73, 940 (1948). Prepn of metal: S. Fried, N. Davidson, J. Am. Chem. Soc. 70, 3539 (1948); L. B. Magnusson, T. J. LaChapelle, ibid. 3534. Presence in nature: Seaborg, Perlman, ibid. 70, 1571 (1948). Chemical properties: Seaborg, Wahl, ibid. 1128. Reviews: C. Keller, The Chemistry of the Transactinide Elements (Verlag Chemie, Weinheim, English Ed., 1971) pp 253-332; W. W. Schulz, G. E. Benedict, Neptunium-237; Production and Recovery, AEC Critical Review Series (USAEC, Washington, D.C., 1972) 85 pp; Comprehensive Inorganic Chemistry vol. 5, J. C. Bailar, Jr. et al., Eds. (Pergamon Press, Oxford, 1973) passim; J. A. Fahey in The Chemistry of the Actinide Elements vol. 1, J. J. Katz et al., Eds. (Chapman and Hall, New York, 1986) pp 443-498; G. T. Seaborg in Kirk-Othmer Encyclopedia of Chemical Technology vol. 1 (Wiley-Interscience, New York, 4th ed., 1991) pp 412-444.
Properties: Silvery metal; develops a thin oxide layer upon exposure to air for short periods. Reacts with air at high temperatures to form NpO2. Exhibits 3 allotropic modifications: orthorhombic a-form, d 20.45, transforms to b-form at 280°; tetragonal b-form, d 19.36, transforms to g-form at 577°; cubic g-form transforms to liquid at mp 637°. Extrapolated bp 4174°. Np has been obtained in its five oxidation states in soln; the most stable is the pentavalent state. Tetravalent Np is readily oxidized to the hexavalent state by permanganate in the cold, or by strong oxidizing agents; on electrolytic reduction in an atmosphere of nitrogen, the trivalent form is obtained.
Melting point: mp 637°
Boiling point: bp 4174°
Density: d 20.45; d 19.36
CAUTION: Radiation hazard; handling requires special equipment and shielding facilities (Katz et al., loc. cit. vol. 2, p. 1128).
Use: Source material for prodn of 238U (power source).

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