neptunium, radioactive chemical element; symbol Np; at. no. 93; at. wt. 237.0482; m.p. about 640°C;; b.p. 3,902°C; (estimated); sp. gr. 20.25 at 20°C;; valence +3, +4, +5, or +6. Neptunium is a ductile, silvery radioactive metal. It is a member of the actinide series in Group 3 of the periodic table. Neptunium has three distinct forms (see allotropy); the orthorhombic crystalline structure occurs at room temperature. Neptunium forms numerous chemical compounds. The element was discovered in 1940 by Edwin M. McMillan and Philip H. Abelson, who produced neptunium-239 (half-life 2.3 days) by bombarding uranium with neutrons from a cyclotron at the Univ. of California at Berkeley. Neptunium, the first transuranium element, was named for the planet Neptune, which is beyond Uranus in the solar system. Neptunium is found in very small quantities in nature in association with uranium ores. There are 20 known isotopes of neptunium. Neptunium-237, the most stable, has a half-life of 2.14 million years and is used in neutron-detection equipment.