A stable elementary particle in the lepton class having a negative electric charge of 1 elementary unit (about 1.602 × 10^-19 coulombs) and a mass of about 9.11 × 10^-28 grams. Electrons are found in shells orbiting the nuclei of atoms and can also move freely through space as cathode rays in a cathode-ray tube or as beta particles emitted by radioactive nuclei, or flow in a current through a conducting material impelled by an electric potential difference.