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Structure of the nucleus and the strong nuclear force

As you know there are two kinds of particle in the nucleus of an atom, protons, carrying a unit positive charge and neutrons which are uncharged. It is therefore pretty surprising that the nucleus holds together at all - you would expect the electrostatic repulsion from all those positively charged protons to blow it apart. The fact that this doesn't happen is very good evidence for the existence of another attractive force between the nucleons.

This is called the strong nuclear force. It only acts over very short distances (10- 15 to 10-14 m), and it is what holds the nucleus together.

In small nuclei the strong force from all the nucleons reaches most of the others in the nucleus but as we go on adding protons and neutrons the balance becomes much finer. The longer range electrostatic force affects the whole nucleus but the short-range strong nuclear force of any particular nucleon only affects those nucleons around it - the rest of the nucleus is unaffected. The nucleons are not held together so tightly and this can make the nucleus unstable. (See nuclear fission).

However the more protons there are in a nucleus the greater the electrostatic forces between them and we need a few extra neutrons to help "keep the protons apart". This is why heavy nuclei have more neutrons than protons.

The variation of neutron number with proton number is shown in the graph. You can see that for light elements these two numbers are the same but become very different for heavy elements. Adding more neutrons helps to keep the nucleus stable but when the number of protons is greater than 83 adding more neutrons is not enough and all elements with a proton number of greater than 83 are unstable.

Nuclei that lie on or near the central line are stable but as you move to either side of that region they become unstable.
They can return to the stable state by emitting either a positron (side B) or an electron (side A).

© Keith Gibbs 2011