Neptunium is the chemical element with atomic number 93. Its isotope neptunium-239 occurs between uranium-239 and plutonium-239 in a neutron-capture and beta decay production chain.
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Neptunium is a chemical element with the symbol Np and atomic number 93. It is a silvery, radioactive metal that belongs to the actinide series of the periodic table. Neptunium is notable for being the first transuranium element, meaning it was the first element discovered that has an atomic number greater than that of uranium (92). Neptunium was discovered in 1940 by Edwin McMillan and Philip H.