protones

Protones (protons) are positively charged subatomic particles that reside in atomic nuclei. They carry a charge of +1 elementary charge and have a rest mass of about 1.6726 × 10^-27 kilograms (approximately 938.27 MeV/c^2). Protons have intrinsic angular momentum (spin) 1/2 and are fermions.

Protons are not elementary; they are baryons composed of three valence quarks—two up quarks and one down quark—bound together by the strong force via gluons. This internal structure gives rise to properties such as the proton’s electric charge distribution and magnetic moment. The proton is part of an isospin doublet with the neutron.

In atomic nuclei, protons contribute to the atomic number Z, which determines the identity of the element and, together with neutrons, provides nuclear binding energy. The proton’s electric charge also governs electromagnetic interactions and chemical behavior through the effect on the electron cloud. Free protons are stable, with a lifetime exceeding the age of the universe; processes beyond the Standard Model could change that. Protons are released in particular during fusion and various high-energy reactions.

Discovery and terminology: The hydrogen nucleus was identified as a distinct particle in early 20th-century experiments. In 1919, Rutherford proposed the name proton for this positively charged nucleus, a term that became standard. Protons are produced in particle accelerators and cosmic-ray interactions, and they are routinely observed in detectors and colliders.

In stars, proton fusion drives energy production through the proton–proton chain, a key step in stellar nucleosynthesis.