A delicate balance. In summary, nuclei are held together by a delicate balance among {a} the attractive strong force between nucleons, {b} the electrical repulsion between protons, {c} the dilution of this repulsive force by neutrons, and {d} the instability created by the Exclusion Principle if there are too many neutrons. Thus, for a given number of protons there is a narrow range of allowable neutron numbers: not enough and the protons will fly apart because of electrical repulsion: too many and the nucleus will be unstable because of the Exclusion Principle.
( Rodney A. Brooks )
[ Fields of Color: The theory ]
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