The stability of an isotope is based on the ratio of neutrons to protons in the nucleus. If the ratio is too high or too low, the nucleus will be unstable and will decay. The ideal ratio of neutrons to protons varies depending on the element. For example, helium-4 has two protons and two neutrons, which is a stable ratio. However, helium-3 has two protons and one neutron, which is an unstable ratio. Helium-3 will decay into helium-4 by emitting a beta particle.