protonneutron

A proton and a neutron are nucleons, the subatomic particles that reside in atomic nuclei. Protons carry a positive electric charge, equal in magnitude to the electron’s charge, while neutrons are electrically neutral. They have very similar masses, around one atomic mass unit, and both have half-integer spin, classifying them as fermions.

In terms of quark composition, the proton consists of two up quarks and one down quark (uud), whereas the neutron is made of one up quark and two down quarks (udd). This quark structure explains their charges and magnetic moments. Protons and neutrons together form the vast majority of the visible mass of ordinary matter. Their collective properties give nuclei their stability and determine the behavior of atoms.

The interaction between protons and neutrons in nuclei is mediated by the strong nuclear force, a residual interaction that is much stronger than gravity but acts only over very short ranges, on the order of a femtometer. This force binds protons and neutrons into nuclei, producing nuclear binding energy that stabilizes matter. The balance between protons and neutrons, or the neutron-to-proton ratio, influences nuclear stability and affects which isotopes are long-lived. Nuclei can undergo beta decays that convert neutrons into protons or vice versa, mediated by the weak interaction.

Protons and neutrons are produced in the early universe through nucleosynthesis and in stars by fusion processes. They determine the identity of atoms (the element) and, through nuclear reactions, drive stellar energy production and the chemical evolution of the cosmos.