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938.272013(23)&_160;MeV/c2 1.007276466(10)&_160;u [1] Protons are spin-1/2 fermions and are composed of three quarks[2], making them baryons. The two up quarks and one down quark of the proton are held together by the strong force, mediated by gluons. Protons and neutrons are both nucleons, which may be bound by the nuclear force into atomic nuclei. The nucleus of the most common isotope of the hydrogen atom is a single proton (it contains no neutrons). The nuclei of heavy hydrogen (deuterium and tritium) contain neutrons. All other types of atoms are composed of two or more protons and various numbers of neutrons. The number of protons in the nucleus determines the chemical properties of the atom and thus which chemical element is represented; it is the number of both neutrons and protons in a nuclide which determine the particular isotope of an element. Protons are observed to be stable and their theoretical minimum half-life is 1×1036 years. Grand unified theories generally predict that proton decay should take place, although experiments so far have only resulted in a lower limit of 1035 years for the proton's lifetime. In other words, proton decay has never been witnessed and the experimental lower bound on the mean proton lifetime (2.1×1029&_160;years) is put by the Sudbury Neutrino Observatory[3].
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