Neptunium-239 is a radioactive isotope of the element neptunium, which is part of the actinide series in the periodic table. It has a half-life of about 2.356 days, meaning it takes this amount of time for half of a sample to decay. Neptunium-239 is produced through the neutron capture process, often in nuclear reactors. This isotope decays into plutonium-239 through beta decay, making it significant in the context of nuclear chemistry and energy. Neptunium-239 is not found naturally in large quantities and is primarily of interest for its potential use in nuclear fuel and research applications.