ELECTRON

lightest stable subatomic particle known. It carries a negative charge which is considered the basic charge of electricity. An electron is nearly massless. It has a rest mass of 9.1 10-28 gram, which is only 0.0005 the mass of a proton. The electron is a fermion, a type of particle named after the Fermi-Dirac statistics that describe its behaviour. It has a half-integral spinspin constitutes the property of intrinsic angular momentum in quantum-mechanical terms. Moreover, the electron reacts only by the electromagnetic, weak, and gravitational forces; it does not respond to the short-range strong nuclear force that acts between quarks and binds protons and neutrons in the atomic nucleus. The electron has an antimatter counterpart called the positron. This antiparticle has precisely the same mass and spin, but it carries a positive charge. If it meets an electron, both are annihilated in a burst of energy. Positrons are rare on the Earth, being produced only in high-energy processes (e.g., by cosmic rays) and live only for brief intervals before annihilation by electrons that abound everywhere. The electron was the first subatomic particle discovered. It was identified in 1897 by the British physicist J.J. Thomson during investigations of cathode rays. His discovery of electrons, which he initially called corpuscles, played a pivotal role in revolutionizing knowledge of atomic structure. Under ordinary conditions, electrons are bound to the positively charged nuclei of atoms by the attraction between opposite electric charges. In a neutral atom the number of electrons is identical to the number of positive charges on the nucleus. Any atom, however, may have more or fewer electrons than positive charges and thus be negatively or positively charged as a whole; these charged atoms are known as ions. Not all electrons are associated with atoms. Some occur in a free state with ions in the form of matter known as plasma. Within any given atom, electrons move about the nucleus in an orderly arrangement of orbitals, the attraction between electrons and nucleus overcoming repulsion among the electrons that would otherwise cause them to fly apart. These orbitals are organized in concentric shells proceeding outward from the nucleus with an increasing number of subshells. The electrons in orbitals closest to the nucleus are held most tightly; those in the outermost orbitals are shielded by intervening electrons and are the most loosely held by the nucleus. As the electrons move about within this structure, they form a diffuse cloud of negative charge that occupies nearly the entire volume of the atom. The arrangement of electrons therefore determines the size of an individual atom. It also affects the way that the atom responds to other atoms, particles, and electromagnetic radiation.