Some elements are unstable and undergo radioactive decay . The unstable elements usually have too many or too few neutrons in their nuclei and are known as radioactive isotopes . When an isotope is radioactive it emits particles or energy in an effort to become stable. It will usually achieve stability by changing into another element. This occurs predominantly with elements which contain a greater number of protons than lead.
Fusion can take place between isotopes of elements lighter than iron.
In both instances the result is a different chemical element which may or may not be stable. Both fusion and fission can release vast amounts of energy because the mass of the resultant particles is less than the sum of the masses of the original particles. It is this difference in mass, known as the mass defect , that is converted into energy.