The isotopes of a particular element have the same number of protons in the
nucleus but a different number of neutrons, giving them the same proton
number (atomic number) but a different nucleon number (mass number, i.e.
number of protons + number of neutrons). Isotopes may be stable or
radioactive. Radioactive isotopes (radioisotopes) disintegrate spontaneously
at random to yield radiation and a decay product.