nucleon

A nucleon is a collective term for the proton and the neutron, the fermionic constituents of atomic nuclei. Each nucleon has spin 1/2 and a baryon number of 1. The proton carries electric charge +1, while the neutron is electrically neutral. Their masses are about 938 MeV/c^2 for the proton and 939.6 MeV/c^2 for the neutron. Nucleons are often treated as an isospin doublet, related by the interchange of up and down quarks.

In terms of internal structure, a nucleon is a bound state of three valence quarks (proton: two up quarks and one down quark; neutron: one up quark and two down quarks) held together by the strong interaction described by quantum chromodynamics. The quark content is supplemented by a sea of quark–antiquark pairs and gluons that contribute to mass and other properties. Nucleon structure is probed through form factors and structure functions in scattering experiments.

Within atomic nuclei, nucleons interact via the short‑range nuclear force, which binds protons and neutrons together. This force operates over roughly 1 femtometer and includes two‑ and three‑nucleon components. The typical binding energy per nucleon in stable nuclei is about 8 MeV, and nuclear size scales approximately with the atomic mass number as A^(1/3).

Historically, nucleons emerged as the fundamental constituents of the nucleus following discoveries of the proton and neutron in the early 20th century. Today, nucleons are central to both nuclear and particle physics and are studied by elastic and inelastic scattering to reveal their internal structure and interactions.