Neptunium

Neptunium is a chemical element with the symbol Np and atomic number 93. A radioactive metal, neptunium is the first transuranic element, and belongs to the actinide series. Its most stable isotope, 237Np, is a by-product of nuclear reactors and plutonium production, and it can be used as a component in neutron detection equipment. Neptunium is also found in trace amounts in uranium ores due to transmutation reactions.

The periodic table of Dmitri Mendeleev published in the 1870s showed a " — " in place after uranium similar to several other places for at that point undiscovered elements. Also, a 1913 publication of the known radioactive isotopes by Kasimir Fajans shows the empty place after uranium.