Shell model of the nucleus is used to explain stability of some atomic nuclides having magic numbers of protons or/and neutrons.
Nucleons in the nucleus are arranged into shells. When a shell is fully filled with the nucleons we say that nuclei is stable. Magic number is a number of nucleons ( either protons or neutrons, separately) such that they are arranged into complete shells within the atomic nucleus. The magic munbers are : 2, 8, 20, 28, 50, 82, 126.
The stability of the nucleus with magic numbers is due to a large energy gap between a series of filled evergy levels and the next level, which is empty. Such large gaps are said to separate shells, althogh these shells are not as clearly linked to the spartial structure of the nucleus as electron shells are to their orbits.