Neptunium (Np) is a radioactive, silvery, metallic element, atomic number 93. Neptunium is the first of the transuranic elements, having a number of isotopes with mass numbers from 231 to 241 and half-lives ranging from 7.3 min to 2.2 million years (the latter in the case of ²³⁷Np). It is found in trace quantities in uranium ores and resembles uranium in its chemical properties. is produced synthetically by nuclear reactions. Neptunium was first synthesized in 1940 and is made in breeder reactors as a by-product of plutonium production by neutron irradiation of uranium-238.