neutron

The electrically neutral nucleon, a baryon composed of two down quarks and one up quark, which has a mass 1,839 times that of an electron, is stable when bound in an atomic nucleus, but has a mean lifetime of 886 seconds as a free particle. It is a basic component of all atomic nuclei except the protium isotope of hydrogen.

A subatomic particle forming part of the nucleus of an atom and having no charge; it is a combination of an up quark and two down quarks

an elementary particle with 0 charge and mass about equal to a proton; enters into the structure of the atomic nucleus