neutron

Science Dictionary
neutron   (n'trŏn')  Pronunciation Key 
An electrically neutral subatomic particle in the baryon family, having a mass of 1.674 × 10-24 grams (1,838 times that of the electron and slightly greater than that of the proton). Neutrons are part of the nucleus of all atoms, except hydrogen, and have a mean lifetime of approximately 1.0×103 seconds as free particles. They consist of a triplet of quarks, including two down quarks and one up quark, bound together by gluons. In radioactive atoms, excess neutrons are converted to protons by beta decay. Beams of neutrons from nuclear reactors are used to bombard the atoms of various elements to produce fission and other nuclear reactions and to determine the atomic arrangements in molecules. See Table at subatomic particle.
The American Heritage® Science Dictionary
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