All of the atoms of a particular element have the same atomic number, but they can have different numbers of neutrons (i.e., different mass numbers). isotopes of a chemical element consist of atoms having different numbers of neutrons. When an atomic nucleus is unstable it decomposes or decays spontaneously, emitting high-energy particles. The emissions from radioactive decay, called radiation, can consist of electrons (beta particles), or electromagnetic energy in the form of photons, or helium ions (alpha particles). The process of decay can produce a product that is itself unstable, in which case it too will decay. The process continues until a stable nuclide is finally formed.