The motions of the particles within a nucleus will be the factor its properties. They cannot be calculated precisely, so physicists have invented simplified models from which properties can be predicted. One model assumes that the nucleus is equivalent to a charged liquid drop. The mass of a given isotope is mainly the sum of the free masses of its nucleons. Corrections are calculated for the nuclear binding, the surface tension of the drop, and the electrostatic repulsion of the protons in the nucleus, taking note also of the excess of neutrons over protons, and whether or not the nucleon number is odd or even. The resulting formula can be modified to fit closely all the measured masses of the known isotopes.

Some nucleus properties are explained by the shell model, which assumes that every nucleon moves in an average field created by all the other nucleons. Its permitted orbits, or energy levels, are next calculated by quantum mechanics. Corrections are made for the spin-orbit force, which depends on whether a nucleon's spin is parallel or opposite to its orbital momentum. The shell model permits the calculation of the energies of many excited levels of nuclei and the way in which the spins and the momentum of the individual nucleons combine to give the net spin of the nucleus.

Energy levels of some nuclei demonstrate patterns of regularity. In the collective model of the nucleus, these properties are sure to be determined by the motion of the nucleus as a whole. At last, the unified model of the nucleus combines the effects of the collective and shell models to provide more comprehensive agreement with experimental observation.

Bookmark This Page