I don't know how nucleons occupy each energy level, especially the one with multiple states. For example, 1p3/2, there are 4 states in this level, that can be occupied 4 nucleons. If there are 2 nucleons in this level, do they must be paired-up? Why? And if there are 3 nucleons in this level, 2 nucleons are paired and 1 nucleon is unpaired? Why? According to Pauli exclusion principle, two fermions cannot occupy the same quantum state. For 1p3/2, how can I assign nucleons to this state without violating this principle?