Neutron emission is a type of radioactive decay of atoms containing excess neutrons, in which a neutron is simply ejected from the nucleus. Neutron emission is one of the ways an atom reaches its stability. An atom is unstable, therefore radioactive, when the forces in the nucleus are unbalanced. The instability of the nucleus results from the nuclei having extra neutrons or extra protons. Two examples of isotopes that emit neutrons are beryllium-13 (mean life 6979270000000000000♠2.7×10 s) and helium-5 (6978700000000000000♠7×10 s). Commonly, it is abbreviated with a lower case n.