neutron, An electrically neutral (uncharged) subatomic particle in the baryon family, having a mass 1,839 times that of the electron, stable when bound in an atomic nucleus, and having a mean lifetime of approximately 1.0×10³ seconds as a free particle. The neutron and the proton form nearly the entire mass of atomic nuclei, so they are both called nucleons.
[NEUTR(AL) + -ON.]

The American Heritage® Dictionary of the English Language, Third Edition copyright © 1992 by Houghton Mifflin Company. Electronic version licensed from InfoSoft International, Inc. All rights reserved.

The neutron was discovered by James Chadwick in 1932. The stable isotopes of all elements except hydrogen and helium contain within the nucleus a number of neutrons equal to or greater than the number of protons. The preponderance of neutrons becomes more marked for very heavy nuclei. A neutron bound within the nucleus may be stable. A nucleus with an excess of neutrons, however, is radioactive; the extra neutrons (as well as any free neutrons not bound within a nucleus) convert by beta decay into a proton, an electron, and an antineutrino. The antineutron, the neutron's antiparticle, was discovered in 1956. The neutron is made up of still smaller particles called quarks. See elementary particles.

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