The known elementary matter particles of the Standard Model fall into two families: quarks and leptons.

There are six quark flavors: up, down, charm, strange, top and bottom. Quarks carry fractional electric charge and a strong-interaction charge called color. Isolated quarks are not observed at ordinary energies; they bind into hadrons.

A baryon contains three valence quarks. For example, $$p=uud,\qquad n=udd.$$ Using quark charges $Q_u=+2e/3$ and $Q_d=-e/3$ gives $$Q_p=2\left(\frac{2e}{3}\right)-\frac e3=+e,$$ $$Q_n=\frac{2e}{3}-2\frac e3=0.$$ A meson contains a quark-antiquark pair.

The six leptons are the electron, muon and tau, together with their associated neutrinos. Charged leptons participate in electromagnetic and weak interactions; neutrinos are electrically neutral and interact through the weak interaction and gravity.

Every matter particle has a corresponding antiparticle with opposite additive charges. Particle reactions can transform flavors subject to interaction-specific conservation laws, but charge, energy and momentum remain conserved.

The quark-lepton classification replaces the older picture in which every proton, neutron and meson was elementary. Hadrons are composite quantum states of quarks and gluons; leptons, as far as current experiments show, have no internal substructure.