The helium nucleus, for example, contains two protons and two neutrons. If the neutrons were not there, the electrical repulsion between the protons would overcome the strong binding force. However if two neutrons are also present, the binding force is increased {because there are now four nucleons}, while the electrical repulsion between the protons is weakened because they are not as close together. The result is a stable helium nucleus-also known as an alpha particle {the same radiation used by Rutherford in his gold foil experiment}. If more protons are present, as in the heavier elements, more neutrons are needed to dilute the increasingly large repulsive force. This is why in most atoms the neutrons outnumber the protons. However if there are too many neutrons the nucleus will break down for a reason known as the Pauli Exclusion Principle {See Chap. 6}.
( Rodney A. Brooks )
[ Fields of Color: The theory ]
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