A charge-neutral particle (of the hadron type) which is one of the two particles that make up the nuclei of atoms. Neutrons are unstable outside the nucleus, but stable within it. The number of protons in the nucleus determines what element that nucleus is. Different isotopes of a given element have different numbers of neutrons in the nucleus. The total number of neutrons and protons affects properties such as radioactivity or stability, the types of nuclear reactions, if any, in which the isotope will participate, and so forth.