Discovered | Name | Characteristics | Found | Uses

Neptunium is a member of the actinide series of elements, which stretches from actinium (Ac) to lawrencium (Lr). It is the first transuranium element, all of which must be made artificially.

Early in this century it was assumed that the periodic table ended with element 92, uranium. However, in 1932 James Chadwick discovered the neutron. This was followed by the realization that, if the nucleus of an element could be made to absorb a neutron, and then the unstable atom emitted a beta particle, a new nucleus would have been formed. With this in mind, E. Fermi and others tried to synthesize element 93 by bombarding 238U with neutrons. Unfortunately, after searching for some time, no evidence of a new element beyond uranium was found. Instead, they found that the uranium had split into elements of smaller nuclei; they had discovered nuclear fission.

Nonetheless, in early 1940 Edwin McMillan at the University of California thought that he had found evidence for element 93 among the fission products of uranium, an observation that was later confirmed by K. Starke, F. Strassmann, and O. Hahn in Germany. The new element was named neptunium because it follows uranium in the periodic table, just as the planet Neptune follows the planet Uranus in the solar system.

Since Np was found in the debris from the neutron bombardment of uranium

238U + 1n 239U 239Np + b

it was assumed that the element could only be made in the laboratory. However, it has since been found that there are traces in naturally occurring uranium ores. Today neptunium is made in kilogram quantities as a byproduct of breeder reactors.

Neptunium compounds exist with Np in oxidation states of +2 to +7, with +5 probably being most stable in aqueous solution. The known oxides are Np2O5 (dark brown), NpO2 (brown-green), and NpO. In addition, there are a number of halides known. For example, just as uranium and plutonium form hexafluorides, so does neptunium. In the case of Np the compound NpF6 is a volatile solid with a melting point of 54.7°C.

At least 15 isotopes of Np are known, all radioactive. The most stable is 237Np with a half-life of 2.14 million years.

There are no known uses of neptunium.