Quark structure of the carbon nucleus. Computer visualisation of the nucleus of a carbon atom. The most common isotope, carbon-12, consists of six protons and six neutrons. These are represented as triplets of quarks surrounded by quantum clouds of gluons. Protons have two 'up' quarks (blue) and one 'down' quark (red); neutrons have one 'up' and two 'down'. 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.

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