1. An uncharged elementary particle that has a mass nearly equal to that of the proton and is present in all known atomic nuclei except the hydrogen nucleus.
2. A neutral hadron that is stable in the atomic nucleus but decays into a protron, an electron and an antineutrino with a mean life of 12 minutes outside the nucleus. Neutronics exist in all atomic nuclei except normal hydrogen. Reported in 1932 by James Chadwick.