A particle of matter that has a negative electric charge of 4.8 E^-10 esu and a mass of 9.1E^-28g or 1/1837 the mass of a proton. They can be found as a constituent part of an atom orbiting around the nucleus or in the free state. Electrons are arranged around the nucleus of an atom in from one to seven orbitals with the number of electrons in each orbital is strictly limited by the laws of physics.