Quark structure of the neutron. Computer visualisation of the structure of the neutron, one of the building blocks of atomic nuclei. The three objects seen here are quarks, held together by the strong nuclear force. This force is mediated by the exchange of virtual gluons which form a quantum cloud around the quarks. The colours here represent the quark flavour"; the proton is made of two "down" quarks (pink) and one "up" quark (green). These quarks have electric charges of -1/3 and +2/3 respectively, so the neutron has no net electric charge (is electrically neutral)."