Science / Periodic Table of Elements / Neptunium (Np): Atomic number: 93, Atomic mass: (237) g.mol -1, Electronegativity: 1.3, Density: 20.25 g.cm-3 at 20Â°C, Melting point: 640 Â°C, Boiling point: 3902 Â°C, Vanderwaals radius: unknown, Ionic radius: unknown, Isotopes: 8, Discovered by: McMillan in 1940. Neptunium is a ductile, silvery, radioactive metal. Neptunium forms numerous chemical compounds. Chemically it is extremely reactive and is attacked by oxygen, steam and acids, but not by alkalis. It can exist in many oxidation states, from neptunium (II) to neptunium (VII). Applications: Neptunium has been used in neutron detectors. It does not have any commercial application. Neptunium in the environment: Neptunium occurs naturally in Earth, being present in minute quantities in uranium ores. Today neptunium-237 is extracted in kg quantities from the spent uranium fuel rods of nuclear reactors.