The nucleus of an atom is identified by the number of protons it contains. Atoms of the same element have the same number of protons, but may have different numbers of neutrons, making them isotopes of that element. The strong nuclear force holds the protons and neutrons together in the nucleus despite their positive charges. Some nuclei are unstable if they have too many or too few neutrons. These unstable nuclei undergo radioactive decay, releasing particles or energy, to become more stable. This can result in the element changing, known as transmutation.