Nuclear shell model

The nuclear shell model is a theoretical construct in nuclear physics which describes that the nucleus of an atom can exist in discrete energy states. This model is partially analogous to the electron orbitals of the atomic shell model.

The ground state is the lowest energy state for a specific nucleus, and represents its most stable configuration.

When the nucleus has excess energy, it can leave the ground state and inhabit excited states, which are most often short-lived because they are associated with relative instability.

An excited, unstable nucleus with excess energy is considered radioactive, and will emit radiation (through alpha, beta or gamma decay) when it spontaneously transitions to a more stable energy state.