Definition of Transuranic element

Of the elements with atomic numbers 1 to 92, all but four (technetium, promethium, astatine, and francium) occur in easily detectable quantities on earth, having stable, or very long half life isotopes, or are created as common products of the decay of uranium.

All of the elements with higher atomic numbers, however, have been first discovered artificially, and other than plutonium and neptunium, none occur naturally on earth. They are all radioactive, with a half-life much shorter than the age of the Earth, so any atoms of these elements, if they ever were present at the earth's formation, have long since decayed. Trace amounts of neptunium and plutonium form in some uranium-rich rock, and small amounts are produced during atmospheric tests of atomic weapons. The Np and Pu generated are from neutron capture in uranium ore with two subsequent beta decays (²³⁸U → ²³⁹U → ²³⁹Np → ²³⁹Pu).