Radioactive metallic transuranic element, belongs to the actinoids. Np237, the most stable isotope, has a halflife of 2.2*10^6 years and is a by product of nuclear reactors. The other known isotopes have mass numbers 229 through 236, and 238 through 241. Np236 has a halflife of 5*10^3 years. First produced by Edwin M. McMillan and P.H. Abelson in 1940.
