a silvery metallic transuranic element synthesized in the production of plutonium and occurring in trace amounts in uranium ores. Symbol: Np; atomic no: 93; half-life of most stable isotope,237Np: 2.14 × 106 years; valency: 3, 4, 5, or 6; relative density: 20.25; melting pt: 639±1°C; boiling pt: 3902°C (est)