Neptunium (named for the planet Neptune) was the first of the transuranium elements to be synthesized (photo above is of the oxide). The synthesis took place at Berkeley, California, after initial examination of the decay products of U-235 suggested the possibility of a new element. Credit for the discovery goes to McMillan and Abelson in 1940. Although the neptunium on which the characterization work was done was synthesized in a cyclotron, we now know that minute amounts of the element exist in the environment (the longest-lived isotope has a half-life of about 2 million years). All isotopes of the metal are radioactive.
Stephen R. Marsden (ChemTopics)