Definition: A proton is a positively charge particle that resides within the atomic nucleus. The number of protons in the atomic nucleus is what determines the atomic number of an element, as outlined in the periodic table of the elements.

The proton has charge +1 (or, alternately, 1.602 x 10-19 Coulombs), the exact opposite of the -1 charge contained by the electron. In mass, however, there is no contest - the proton's mass is approximately 1,836 times that of an electron.

Discovery of the Proton

The proton was discovered by Ernest Rutherford in 1918 (though the concept had been earlier suggested by the work of Eugene Goldstein). The proton was long believed to be an elementary particle until the discovery of quarks. In the quark model, it is now understood that the proton is comprised of two up quarks and one down quark, mediated by gluons in the Standard Model of quantum physics.

Proton Details

Since the proton is in the atomic nucleus, it is a nucleon. Since it has a spin of -1/2, it is a fermion. Since it is composed of three quarks, it is a triquark baryon, a type of hadron. (As should be clear at this point, physicists really enjoy making categories for particles.)
  • Mass: 938 MeV/c2 = 1.67 x 10-27 kg
  • Charge: +1 fundamental unit = 1.602 x 10-19 Coulombs
  • Diameter: 1.65 x 10-15 m