Neptunium is a chemical element, having the chemical symbol Np. Its atomic number (the number of protons) is 93. It has a standard atomic weight of 237.0462 g•mol −1 and it is a solid in its elemental form.
Neptunium was the first synthetic transuranium element of the actinide series to be discovered. In 1940, Edwin McMillan and Philip H. Abelson produced the isotope 239Np at the Berkeley Radiation Laboratory of the University of California, Berkeley by bombarding uranium with cyclotron-produced, slow moving neutrons.