A chemical element, symbol Np, atomic number 93. Neptunium is a member of the actinide or 5f series of elements. It was synthesized as the first transuranium element in 1940 by bombardment of uranium with neutrons to produce neptunium-239. The lighter isotope ²³⁷Np, a long-lived alpha emitter with half-life 2.14 × 10⁶ years, is particularly important chemically.