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||Named after the planet Neptune.
||221 pm (Van der Waals)
||6, 5, 4, 3
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 1200°C. 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 280°C), tetragonal, density (313°C) 19.36 g/cm3, gamma-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 Pm3+, 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 NpF
3, 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.