The nucleus and the strong nuclear force

The diagram is of a carbon atom. It shows a cloud of electrons (six in this case) orbiting a central nucleus. The nucleus is the part of the atom within the dotted line on the diagram. The nucleus of an atom is very very small – about one hundred million million nuclei put side by side would fit into one centimetre.

The nuclear structure of an atom was discovered by Rutherford and others in 1911

As you know the nucleus of an atom consists of a collection of two types of particle:
(a) protons – small positively charged particles
(b) neutrons – slightly heavier particles than protons but with no charge

Light nuclei such as hydrogen and carbon usually contain as many protons as neutrons but the nuclei of heavy elements such as iron, radium or uranium contain many more neutrons than protons. You can find a list of the structure of many nuclei in

One big problems with this model on a nucleus is why the nucleus 'sticks' together. The protons all have positive charges and the neutrons no charge so why doesn't the nucleus simply burst apart? The positive charges of the protons should make them repel each other – the gravitational force between them being much too small to overcome this electrostatic repulsion.

The explanation is that there is another force that acts only within the nucleus and between particles such as protons and neutrons. This is called the strong nuclear force. Within the nucleus this force is strong enough to overcome the electrostatic repulsion between the protons and so hold the nucleus together.

In small nuclei the strong nuclear force from all the nucleons reaches most of the others in the nucleus but as we go on adding protons and neutrons the balance between the strong force and the electrostatic force 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.

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© Keith Gibbs 2020