The conversion of a chemical element to another, usually by a nuclear reaction or radioactive decay, which changes the number of protons in the nucleus of an atom. Often, a series of subsequent transmutations occurs, where the element undergoes successive radioactive decays in an attempt to achieve the most stable configuration of nucleons in the nucleus. An example of such a sequence of transmutations is the conversion of uranium to plutonium in a breeder reactor. The nucleus of uranium-238 captures a neutron and changes to uranium-239, then this unstable isotope undergoes beta minus decay to change to neptunium-239 (one neutron in its nucleus has decayed to a proton, an electron, and an antineutrino), and this is further transmuted by another beta decay to plutonium-239.