Neptunium is a radioactive actinide metal, the first transuranic element synthesized.
Core Principles & Analysis
Edwin McMillan and Philip Abelson created neptunium at Berkeley in 1940 by bombarding uranium with neutrons, making it the first transuranium element ever produced.
It was named after Neptune, the planet beyond Uranus, extending the planetary naming begun with uranium; plutonium, after Pluto, completed the sequence.
Neptunium-237, its most stable isotope at 2.14 million years, is recovered from spent reactor fuel and irradiated to make plutonium-238, the heat source for deep-space missions.