neptunium (nĕptōˈnēəm) [key], radioactive chemical element; symbol Np; at. no. 93; mass number of most stable isotope 237; 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. 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. Neptunium forms numerous chemical compounds, and is found in very small quantities in nature in association with uranium ores.
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.