Neptunium is one of the transuranium elements in the actinide series of the periodic table.

Neptunium is a silvery metal that exists in at least three different crystalline forms, hence the variations in specific gravity (from 18 to 20).

The element is reactive and shows four ionic oxidation states.

It was discovered in 1940 by the American physicists Edwin M. McMillan and Philip H. Abelson. It is produced by bombardment of uranium-238 with neutrons; the resultant uranium-239 decays radioactively by emitting a beta particle to form neptunium-239.

The neptunium isotope in turn emits a beta particle, forming the important isotope plutonium-239, one of the materials of which atomic bombs are made. 

Isotopes of neptunium with mass numbers from 228 to 242 are known. The most stable, neptunium-237, has a half-life of 2.14 million years. It was discovered by the American chemists Glenn T. Seaborg and Arthur C. Wahl. This long-lived isotope served as a useful research tool in the atomic bomb project and is used in studies of chemical reactivity.

Neptunium occurs in nature in trace amounts in uranium ores but is produced artificially. It is used as a component in neutron detection devices. 

Neptunium melts at about 630° C (about 1166° F), and boils at about 5235° C (about 9455° F). The atomic weight of neptunium is 237.0482.