Protons

Protons are positively charged subatomic particles that form a key part of atomic nuclei, together with neutrons. They have a rest mass of about 1.6726 × 10^-27 kilograms (approximately 938 MeV/c^2) and an electric charge of +e (about +1.602 × 10^-19 coulombs). Protons possess intrinsic angular momentum (spin) of 1/2 and a magnetic moment. Their effective size, or charge radius, is on the order of 0.84 to 0.87 femtometers.

Protons are baryons, meaning they are made of three quarks bound by gluons through the theory of quantum chromodynamics. The valence quark content of a proton is two up quarks and one down quark (uud). Most of the proton’s mass and dynamics arise from the energy of the strong interaction rather than the rest masses of the constituent quarks.

In atoms, the number of protons in the nucleus, known as the atomic number Z, determines the identity of the element. Nuclei contain varying numbers of protons and neutrons, collectively called nucleons. Protons and neutrons are held together by the strong nuclear force, which overcomes the electromagnetic repulsion between protons. The combination of Z and the neutron number defines isotopes of an element.

Protons are produced in various high-energy processes, most notably in stellar fusion such as the proton-proton chain that powers stars. In ordinary matter, free protons are extraordinarily stable and have a lifetime longer than the age of the universe according to the standard model. In laboratories, protons are accelerated and collided to study fundamental interactions, revealing properties such as charge distribution and form factors.

Historically, the proton was identified as the hydrogen nucleus by Rutherford in the early 20th century, and the term proton was adopted shortly afterward.