Q.What are the constituent particles of a neutron and a proton?

Until roughly the 1960s, protons, neutrons and electrons were all believed to be genuinely fundamental "building blocks" of matter, with nothing smaller inside any of them. In 1964, physicists Murray Gell-Mann and George Zweig independently and theoretically proposed that protons and neutrons are, in fact, not fundamental at all, but are themselves composed of smaller constituent particles that came to be called quarks - a prediction confirmed experimentally in 1968 at the Stanford Linear Accelerator Center (SLAC) in the USA.

There are six known "flavours" of quark - up, down, charm, strange, top and bottom - each with a corresponding antiparticle, and every quark carries an electric charge that is a simple fraction of the elementary charge $e$: the up quark carries charge $+\tfrac23 e$, and the down quark carries charge $-\tfrac13 e$. According to the quark model, a proton is built from two up quarks and one down quark, giving a net charge of $\tfrac23e+\tfrac23e-\tfrac13e=+e$, matching the proton's known charge exactly, while a neutron is built from one up quark and two down quarks, giving a net charge of $\tfrac23e-\tfrac13e-\tfrac13e=0$, matching the neutron's electrical neutrality precisely. …