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| Atomic Number: | 93 |
| Atomic Radius: | 131 pm |
| Atomic Symbol: | Np |
| Melting Point: | 644 șC |
| Atomic Weight: | 237.0482 |
| Boiling Point: | 3900 șC |
| Electron Configuration: | [Rn]7s25f46d1 |
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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 1200oC. 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
280oC), tetragonal,
density (313oC) 19.36 g/cm3,
gamma-neptunium (above 577oC), cubic, density (600oC) 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.
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