Inside a nucleus there are protons and neutrons but what are inside these? Protons and neutrons consist of smaller particles called quarks.
A quark is a fundamental particle. This means it isn’t built up of anything, it is elementary. Protons and neutrons are both built up from two different quarks, up quarks and down quarks.
As you can see from the diagram each nucleon consists of three different quarks. A proton contains two up and one down quark. A neutron contains one up and two down quarks.
Quarks are charged particles. Up quarks are positive and down quarks are negative. We know that a neutron does not have any net charge so how does this work? The sum of the charges of the quarks must equal to zero. Some simple maths may help here.
2d + u = 0 (charge of the neutron)
d + 2u = 1 (charge of the proton)
By using these two equations and substituting one into the other you will find that:
d = -1/3e , u = +2/3e
Different combinations of quarks can be formed but the rule is that the net overall charge has to be a whole number. Protons and neutrons consist of three quarks but other particles can be formed from two quarks, these are called mesons. Quarks have never been observed in isolation, only bound in other particles.
And that is what protons and neutrons are made of!