Radioactive isotopes are unstable because the addition of extra neutrons overcomes the binding energy that normally holds the nucleus together. The isotope releases energy as radiation to stabilize the nucleus.
Isotopes of an element have the same number of protons, but vary in the number of neutrons in the nucleus. The protons in the nucleus have positive charges and naturally repel one another; however, in stable atoms, the binding energy is greater than the forces of repulsion, and the atom remains stable.
Stable atoms generally have the same number of protons and neutrons. Some accommodate one to two additional neutrons in the nucleus and remain stable. However, once the forces of repulsion reach the critical point, the atom becomes unstable. To restore the balance in the nucleus, the atom releases a neutron.
All elements with atomic numbers greater than 83 are radioisotopes and have unstable, radioactive nuclei. Elements with atomic numbers of 83 or less have at least one stable isotope and one unstable isotope. While there are more than 1,000 radioisotopes, only 50 exist in nature. When the isotope loses a neutron, it often becomes a more stable isotope of the same element. However, some isotopes go through the process of transmutation to become a new element.