Soon after the discovery of the neutron by Chadwick in 1932, Heisenberg proposed that nuclei consisted of neutrons and protons bound together by a strong nuclear force. This model avoided the difficulties of the proton plus electron model of the nucleus and was gradually accepted. By the early thirties a considerable number of nuclear masses had been measured by Aston and others using mass spectrometers and it was found that the binding energy per nucleon was approximately constant for all nuclei. The volumes of nuclei had also been determined from scattering experiments to be roughly proportional to the number of nucleons in the nucleus, which implied an approximately constant nuclear density and that the nuclear radius was proportional to A⅓. Numerically the radius of a nucleus is given by R = r0A⅓ fm where r0 ≈ 1.2.

In this chapter a number of models of nuclei are described which account for different aspects of nuclear behaviour. It is shown that for the bulk properties of nuclei a liquid drop model of a nucleus is very useful. However, to understand the spins of nuclei and the occurrence of magic numbers a description of the motion of individual nucleons is required and this is provided by the simple shell model. The existence of large nuclear electric quadrupole moments indicates that nuclei are generally deformed in shape and the generalisation of the simple shell model to account for this is described.