'''Protons''' are one of the two types of [[sub-atomic]] particles that form in the [[nucleus]] of an [[atom]] (together with [[neutron|neutrons]]).<ref>Serway, Beichner: ''Physics for Scientists and Engineers'', 5th edition</ref>
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A proton is made up of two up [[quark|quarks]] and one down [[quark]]. According to the [[Standard Model]] of particle physics, quarks are [[fundamental particles]], meaning that they cannot be split into smaller particles.<ref>Krane: ''Modern Physics'', 2nd edition</ref>