Quarks: Building Blocks of Protons and Neutrons

Quarks are elementary particles in the Standard Model. An ordinary proton has two up valence quarks and one down quark (uud), while a neutron has one up and two down quarks (udd). Gluons, the carriers of the strong interaction, bind them together. Quarks are not observed individually under ordinary conditions: because of confinement, they occur inside hadrons.