Elements are defined by the number of protons in the nuclei of their individual atoms. For example, oxygen atoms always have 8 protons, iron has 26, and so forth. But more than protons make up the nuclei of atoms.
Uncharged particles called neutrons account for more than half of the mass of atoms.
For each element there are different isotopes, atoms with the same number of protons but different numbers of neutrons. For example, carbon atoms always have six protons, but may have six, seven, or eight neutrons. Some isotopes are stable, but others are radioactive. An isotope will be radioactive if its nuclei are unstable. Large atomic nuclei, with more than 83 protons and their associated complement of neutrons, are inherently unstable.