Neutron
Neutron, electrically neutral elementary particle that is part of the nucleus of the atom. Elementary particles are the smallest parts of matter that scientists can isolate. The neutron is about 10-13 cm in diameter and weighs 1.6749 x 10-27 kg. See also Atom . Neutrons and protons bind tightly together to create atomic nuclei. The number of protons an atom contains determines which chemical element it is, ranging from 1 proton for hydrogen to 92 for uranium, the largest naturally occurring element. Each atom usually contains about as many neutrons as protons, but different atoms of the same element may have different numbers of neutrons. Atoms that differ only in the number of neutrons are called isotopes . For example, most atoms of the simplest element, hydrogen, have a nucleus containing only a single proton. In natural hydrogen, however, 0.015 percent of the atoms have a neutron in addition to the proton. This isotope is called heavy hydrogen or deuterium. An element usually has