Nuclear Structure

The light nuclei, like helium, carbon, oxygen, generally have about equal numbers of protons and neutrons. As the nuclei become larger we find a growing excess of neutrons over protons. For example when we get up to Uranium 238, the excess has grown to 146 neutrons to 92 protons.
The most stable isotope of a given element is the one with the lowest possible energy. Because the weak interaction allows protons to change into neutrons and vice versa, the number of protons and neutrons in a nucleus can shift until the lowest energy combination is reached.
Two forms of energy that play an important role in their proess are the extra mass energy of the neutrons, and the electric potential energy of the protons. It takes a lot f to shove two protons together against their electric repulsion. The work you do in shoving them together is stored as electric potential energy which will be released if you let go and the particles fly apart. This energy will not be released, however, if the protons are latched together by the nuclear force. But in that case the electric potential energy can be released by turning one of the protons into a neutron. This will happen if enough electric potential energy is available not only to create the extra neutron rest mass energy, but also the positron required to conserve electric charge.

The reason that the large nuclei have an excess of neutrons over protons is that electric potential energy increases faster with increasing number of protons than neutron mass energy does with increasing numbers of neutrons. The amount of extra neutron rest mass energy is more or less proportional to the number of neutrons. But the increase in electric potential energy as you add a proton depends on the number of protons already in the nucleus. The more protons already there, the stronger the electric repulsion when you try to add another proton, and the greater the potential energy stored. As a result of this increasing energy cost of adding more protons, the large nuclei find their lowest energy balance having an excess of neutrons.