Proton

A proton carries one unit of positive electric charge and has a mass close to that of a neutron. Ordinary hydrogen has a single-proton nucleus. Neutral atoms contain matching proton and electron numbers, but ionization changes electron number without changing the nucleus or the chemical element.

References: [1] [3]

Atomic number counts protons. Changing it changes the element, while changing neutron number produces an isotope. Mass number counts all nucleons and should not be confused with measured nuclear mass. Nuclear binding means that adding individual constituent masses does not directly give a bound nucleus’s mass.

References: [3]

The familiar valence-quark description assigns a proton two up quarks and one down quark. Gluons participate in the strong interaction binding its constituents. This is not a photograph of three stationary beads: sea quarks and dynamical fields also contribute to the proton’s structure.

References: [1] [2]

Scattering experiments probe internal structure by comparing measured distributions with theoretical predictions. Beam energy and detector geometry affect what can be inferred. Proton research thus connects the identity of chemical elements with the strong interaction and the origin of much of the mass in ordinary matter.

References: [2] [1]