Although this weakly radioactive element is found naturally in small quantities in uranium ores, it was discovered in a lab only after a sample of uranium was deliberately bombarded with neutrons in 1940 (see the nuclear reaction in the clue). It has 21 known isotopes with half-lives ranging from between less than a year up to 2.14 million years (for Np237).
Neptunium was named after the planet (and Roman god) Neptune, and uranium was named after the planet (and god) Uranus. Just as Neptune is the next planet out from the Sun after Uranus, neptunium is the next element in the periodic table after uranium.
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