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Neptunium is the chemical element with atomic number 93. Its isotope neptunium-239 occurs between uranium-239 and plutonium-239 in a neutron-capture and beta decay production chain.

Neptunium-239 is a radioactive isotope with ninety-three protons and one hundred forty-six neutrons. It beta-decays to plutonium-239.

It occurs in the sequence following neutron capture by uranium-238 and beta decay of uranium-239. This is an example of transmutation changing the chemical element through a nuclear process, unlike an ordinary chemical reaction that rearranges electrons and bonds.

An unstable uranium isotope produced by neutron capture on uranium-238. Two successive beta-minus decays, through neptunium-239, lead to plutonium-239.