COLLECTIVE MODEL

also called Unified Model, description of atomic nuclei that incorporates aspects of both the shell model and the liquid-drop model to explain certain magnetic and electric properties that neither of the two separately can explain. In the shell model, nuclear structure and behaviour are explained on the basis of unpaired nucleons (protons and neutrons) beyond the passive nuclear core composed of closed shells of paired protons and paired neutrons; in the liquid-drop model, nuclear structure and behaviour are explained on the basis of statistical contributions of all the nucleons (much as the molecules of a spherical drop of water contribute to the overall energy and surface tension). In the collective model, high-energy states of the nucleus and certain magnetic and electric properties (magnetic and quadrupole moments) are explained by the motion of the nucleons outside the closed shells combined with the motion of the paired nucleons in the core. Roughly, the nuclear core may be thought of as a liquid drop on whose surface circulates a stable tidal bulge directed toward the rotating unpaired nucleons outside the bulge. The tide of protons (positively charged particles) constitutes a current that in turn contributes to the magnetic properties of the nucleus, and the greater deformation of the nucleus as the number of unpaired nucleons increases accounts for the measured electric quadrupole moment (which may be considered an index of nuclear shape or a measure of how much the distribution of electric charge in space departs from spherical symmetry).