Neptunium is a synthetic element in the periodic table that has the symbol Np and atomic number 93. A silvery radioactive metallic element, neptunium is the first transuranic element and belongs to the actinide series. Its most stable isotope, 237Np, is a by-product of nuclear reactors and plutonium production and it can be used as a component in neutron detection equipment. Neptunium is also found in trace amounts in uranium ores.
This element has four ionic oxidation states while in solution:

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ChemicalElements.com: Neptunium - Basic information, atomic structure, and isotopes table.
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Neptunium - Data tables and historic information.
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Wikipedia: Neptunium - Notable characteristics, history, occurrence, isotopes, and weapons applications.
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