A proton is a positively charged particle found in the central region of an atom. Alongside neutrons, protons form the atom's nucleus. The arrangement of chemical species in the periodic table reflects how many protons each type of atom contains: that count determines the atom's identity.
Protons and elements
The number of protons in the nucleus is constant for all atoms of a given element, and this number is the atom's atomic number. For example, a single atom of hydrogen usually consists of one proton with one orbiting electron; heavier elements have additional protons paired with neutrons in their nuclei.
Mass and size
Most of an ordinary atom's total mass resides in its nucleus, with protons and neutrons contributing the bulk. A proton's mass is close to one atomic mass unit; a neutron has a similar mass. In comparison, an electron is about 1/1836 the mass of a proton, so the proton dominates atomic mass.
Internal structure and electrical property
Protons are not elementary particles but composite particles made of quarks bound by the strong interaction. Each proton is composed of three valence quarks: two up quarks and one down quark. The fractional electric charge carried by these quarks (+2/3 for each up quark and −1/3 for the down quark) adds up to the proton's overall +1 elementary charge.
The quarks inside a proton, together with the force carriers that bind them, form a dynamic, extended distribution rather than a rigid sphere. Quantum effects give the proton an effective size and an internal structure often described as a cloud of quarks and gluons. The nucleons (protons and neutrons) that populate most nuclei account for the majority of an atom's mass.