Chemical Fact Sheet


Chemical Abstract Number (CAS #) 7439-99-8
Analytical Methods 200.8 - 6020
Atomic Symbol Np

Synopsis from the CRC Handbook of Chemistry and Physics 92nd Edition 2011-2013

Neptunium — (Planet Neptune), Np; at. wt. (237); at. no. 93; m.p. 644 °C; sp. gr. 20.25 (20 °C); valence 3, 4, 5, and 6. Neptunium was the first synthetic transuranium element of the actinide series discovered; the isotope 239Np was produced by McMillan and Abelson in 1940 at Berkeley, California, as the result of bombarding uranium with cyclotron-produced neutrons. The isotope 237Np (half-life of 2.14 × 106 years) is currently obtained in gram quantities as a by-product from nuclear reactors in the production of plutonium. Twenty-three isotopes and isomers of neptunium are now recognized. Trace quantities of the element are actually found in nature due to transmutation reactions in uranium ores produced by the neutrons which are present. Neptunium is prepared by the reduction of NpF3 with barium or lithium vapor at about 1200 °C. Neptunium metal has a silvery appearance, is chemically reactive, and exists in at least three structural modifications: α-neptunium, orthorhombic, density 20.25 g/cm3, β-neptunium (above 280 °C), tetragonal, density (313 °C) 19.36 g/cm3; γ-neptunium (above 577 °C), cubic, density (600 °C) 18.0 g/cm3. Neptunium has four ionic oxidation states in solution: Np+3 (pale purple), analogous to the rare earth ion Pm+3, Np+4 (yellow green); NpO+ (green blue); and NpO++ (pale pink). These latter oxygenated species are in contrast to the rare earths that exhibit only simple ions of the (II), (III), and (IV) oxidation states in aqueous solution. The element forms tri- and tetrahalides such as NpF3, NpF4, NpCl4, NpBr3, NpI3, and oxides of various compositions such as are found in the uranium-oxygen system, including Np3O8 and NpO2.

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