With smaller nuclei, equal numbers of protons and neutrons are very stable. As the nuclei become larger, more stable configurations contain a an excess of neutrons over protons. This causes the slope of the valley of beta stability to decrease. The effect increases toward the heavier nuclei. The heaviest stable nucleus is 209 Bi, which has 126 neutrons, but only 83 protons.
Magic nuclei are relatively more stable than their neighbors.
They said the stability was magic. The phenomenon has been understood since the 1950's, and is related to the spin energy of nucleons, which is much greater than the corresponding energy of orbital electrons in atoms. Return to index of figures and exercises. Return to Cowley's HomePage.