neutron

A neutron is a type of baryon that has no electric charge. (A baryon is a hadron, i.e., a composite particle, that is made up of three quarks.) Neutrons are subject to the strong force, the force which draws together protons and neutrons to form an atomic nucleus. Other than ¹H, all nuclei include neutrons.

Neutrons can be stable when bound to protons, but free neutrons (not so-bound) decay with a half-life of roughly 610 seconds, i.e., mean lifetime of roughly 880 seconds, the free neutron mean lifetime (often shortened to neutron lifetime in context).

Neutrons were created very soon after the Big Bang when the temperature dropped sufficiently that quarks combined into protons and neutrons. Radioactive decay sometimes converts a neutron within a nucleus to a proton or vice versa.