Editing Proton (section)

== Description ==
{{Nuclear physics}}
{{unsolved|physics|How do the quarks and gluons carry the spin of protons?}}
Protons are [[spin-½|spin-{{sfrac|1|2}}]] [[fermion]]s and are composed of three valence quarks,<ref name="Adair1989" /> making them [[baryon]]s (a sub-type of [[hadron]]s). The two [[up quark]]s and one [[down quark]] of a proton are held together by the [[strong interaction|strong force]], mediated by [[gluon]]s.<ref name="Cottingham1986" />{{rp|21–22}} A modern perspective has a proton composed of the valence quarks (up, up, down), the gluons, and transitory pairs of [[sea quark]]s. Protons have a positive charge distribution, which decays approximately exponentially, with a root mean square [[charge radius]] of about 0.8&nbsp;fm.<ref name="Basdevant2005" />

Protons and [[neutron]]s are both [[nucleon]]s, which may be bound together by the [[nuclear force]] to form [[atomic nuclei]]. The nucleus of the most common [[isotope]] of the [[hydrogen atom]] (with the [[chemical symbol]] "H") is a lone proton. The nuclei of the heavy hydrogen isotopes [[deuterium]] and [[tritium]] contain one proton bound to one and two neutrons, respectively. All other types of atomic nuclei are composed of two or more protons and various numbers of neutrons.