Neutrons, protons, where the number of protons determines which atom it is, and the number of neutrons can vary, soms combinations are quite stable, others not so. The nucleus can be described by using a semi-empirical mass formule in which some interactions in the nucleus are modeled, or can be described by a nuclear-shell model in which the state of the nucleons is described with a quantummechanical model resulting in different levels that these nucleons occupy, similar to electon levels in an atom.