Most atoms will live forever if they are stable. Atoms have a fixed number of protons which determines which type of atom it is. Also, atoms contain neutrons, which normally match the number of protons. However, in some cases, the number of neutrons may be different from the number of protons, in which case the atom is unstable and may be radioactive. Radioactive means that the atom can "eject" other particles so that it can become a more stable atom that has the same number of protons and neutrons.