The nuclear shell model is a theoretical framework used to describe the structure of atomic nuclei. It suggests that protons and neutrons, the building blocks of nuclei, occupy specific energy levels or "shells," similar to how electrons are arranged around an atom. These shells are filled according to the principles of quantum mechanics, leading to stable configurations. In this model, the arrangement of nucleons (protons and neutrons) explains various nuclear properties, such as stability and radioactivity. The shell model helps predict the behavior of isotopes and the formation of nuclear magic numbers, which are specific numbers of nucleons that result in particularly stable nuclei.