Because of the complexity of the many-body problem in the atomic nucleus, various models have been suggested to explain various aspects of the entity. In the shell model, the nucleons are regarded as almost independent particles. In the liquid-drop model all the nucleons in the nucleus are regarded as acting collectively in much the same way as the molecules in a liquid. As these models appear to be physically very different it is necessary to distinguish the circumstances under which they are applicable; moreover, to achieve a unified understanding of these different models would be desirable. Considerable insight is provided by considering the interactions between the quasiparticles and collective excitations in the many-nucleon problem. This enables more complicated models, such as the unified model, which incorporate both the shell model and the liquid-drop model, to be constructed. See also interacting-boson model.