| Inside an atomic nucleus there are both protons and neutrons. Protons are generally stable and decay only after extremely long periods of time (on the order of 10^40 years) due to proton decay whereas neutrons have a half-life of about 1936 seconds if left on their own. A proton exchanges a pion with a neutron, hence changing the proton into a neutron and vise versa. This is what allows an atomic nucleus containing neutrons to be stable.
A proton has an internal quark structure uud for a net charge of +1, while a neutron has a quark structure udd for a net charge of 0. The pion has a quark structure ~ud or u~d where ~ signifies an anti-coloured quark. This pion exchange causes a corresponding u within a proton to turn into a d and vice versa in a neutron.
Neutrons are required within a nucleus to stop the protons from repelling from each other due to their positive charge. The electromagnetic force wants to break the nucleus apart, while the strong force wants to hold the nucleus together.
When the number of neutrons exceeds the number of protons by a high enough proportion, a neutron can decay before a proton has the chance to exchange a pion with it, or the nucleus is just too large for the short range of the strong force. And so the neutron decays into a proton, lepton (either an electron, mu or tau) and a photon.
At this point the most likely scenarios are that the nucleus will expel a helium nucleus (an alpha particle),
a lepton or beta particle and a photon, a gamma particle, or break the nucleus into two other nuclei larger than a helium nucleus. The alpha, beta and gamma
particles are the radioactive emissions of a radioactive nucleus. | 
