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Neptunium

Author: Robert Husted, Mollie Boorman

Atomic Number 93
Atomic Symbol Np
CAS ID No. 7439-99-8
Atomic Weight [237] amu
Electron Configuration [Rn] 5f4 6d1 7s2
Melting Point 637.0 C
Boiling Point -
Density 20.400 g/cm3
History

(Planet Neptune) 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 x 106 years) is currently obtained in gram quantities as a by-product from nuclear reactors in the production of plutonium. 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 1200C. Neptunium metal has a silvery appearance, is chemically reactive, and exists in at least three structural modifications: alpha-neptunium, orthorhombic, density 20.25 g/cm3, beta-neptunium (above 280C), tetragonal, density (313C) 19.36 g/cm3, gamma-neptunium (above 577C), cubic, density (600C) 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); NpO2+ (green blue): and NpO2++ (pale pink). These latter oxygenated species are in contrast to the rare earths which 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 the various compositions such as are found in the uranium-oxygen system, including Np3O8 and NpO2. Seventeen isotopes of neptunium are now recognized. The Oak Ridge National Laboratory has 237Np available for sale to its licensees and for export. This isotope can be used as a component in neutron detection instruments.




Last Update: 2011-02-16