Every element has a fixed number of protons inside its nucleus and an equal number of electrons orbiting the nucleus. For example, Hydrogen has one proton and one electron. The nucleus of an element also contains neutrons, the number of which may vary over a wide range. This gives rise to different forms of the same element, called isotopes.

A strong nuclear force holds the protons and neutrons in a nucleus together. Protons are positively charged particles that repel each other. This is because similarly charged particles repel each other. Neutrons are electrically neutral and account for the stability of the nucleus by reducing electrostatic repulsion. Neutrons help bind multiple protons inside the nucleus. As the number of protons increases, additional neutrons are required to form a stable nucleus. For example, the neutron/proton ratio of 3He is 1:2 while that of 238U is greater than 3:2. If there are too many neutrons or too few neutrons present inside a nucleus, the nucleus becomes unstable and may undergo nuclear decay.