The nuclear processes that give rise to chemical elements. There is not one single process that can account for all the elements. The abundance of the chemical elements is determined not just by the stability of the nuclei of the atoms but also how readily the nuclear processes leading to the existence of these atoms occur.

Most of the helium in the universe was produced by fusion in the early universe when the temperature and the pressure were very high. Most of the elements between helium and iron were made in nuclear fusion reactions inside stars. Since iron is at the bottom of the energy valley of stability, energy needs to be put into a nucleus heavier than iron for a fusion reaction to occur. Inside stars some heavy elements are built up by the s-process, where s stands for slow, in which high-energy neutrons are absorbed by a nucleus, with the resulting nucleus undergoing beta decay to produce a nucleus with a higher atomic number. Some heavy elements are produced by the r-process, where r stands for rapid, which occurs in supernova explosions when a great deal of gravitational energy is released when a large star that has exhausted its nuclear fuel collapses to a neutron star, with the release of a very large number of neutrons. A few elements were produced by spallation. Some heavy elements such as gold are made when two neutron stars collide. Elements with a higher atomic number than 92 have been produced artificially. See also nucleosynthesis.