Quark structure of the uranium nucleus. Computer visualisation of the nucleus of a uranium atom. The most common isotope, uranium-238, consists of 92 protons and 146 neutrons. These are represented as triplets of quarks surrounded by quantum clouds of gluons. Protons have two insupins quarks (green) and one insdownins quark (pink); neutrons have one insupins and two insdownins. The clouds of virtual gluons are responsible for mediating the strong nuclear force; this holds the quarks together within the particles and also overcomes the electrical repulsion between adjacent protons.