Neptunium
Neptunium, a radioactive, metallic chemical element. Neptunium metal is silver-white. Neptunium 239, the most important isotope of neptunium, is a short-lived isotope that occurs in the transition of uranium 238 to plutonium 239, an important nuclear fuel. Neptunium was the first of the transuranium elements (elements that are heavier than uranium) to be discovered. In 1940 Edwin M. McMillan and Philip H. Abelson produced it by bombarding uranium with neutrons.
Symbol: Np. Atomic number: 93. Atomic weight: 237 (most stable isotope). Specific gravity: 20.25 Melting point: 1,184 F. (640 C.). Boiling point. about 7,050 F. (3,900 C.). Neptunium has 13 isotopes: Np-229 through Np-241. Neptunium belongs to the actinide series of the Periodic Table and may have a valence of +3, 4, 5, or 6.