At school we are repeatedly told that ‘like charges repel’, this led me to wonder how the protons stayed together in the nucleus of an atom without all repelling one another. The reason for this strange behaviour lies within the field of quantum physics and more specifically quantum chromodynamics, QCD.
To understand how the nucleus of an atom stays together, we first need to understand what protons and neutrons are made up of. They are baryons, meaning that they contain 3 quarks. In the case of a neutron, it will be two down quarks and one up quark and the opposite for a proton. More about this in elementary particles.
It is the interactions between these quarks which is responsible for holding the protons and neutrons together. Each quark is assigned a property called coloured charge: red, green or blue. Only combinations resulting in zero charge or white charge can exist, explaining why quarks cannot exist alone.
In baryons, the only way for white colour to exist is for each quark to have different colour charge. However, these quarks will always be changing colour, in doing so they exchange bosons called gluons, the charge carrier for the strong force.
It is this exchange of gluons which holds the nucleus together, due to the strong force. This force is the strongest of the four fundamental forces, about 1041 times stronger than gravity. However, we do not feel it ourselves because it acts within a tiny range, 10-15 m, about the diameter of the nucleus of an atom.
Electrons are leptons, which means that they do not contain quarks. Therefore they have no colour charge so don’t feel the strong force. They are found outside of the nucleus of the atom, in shells.