A stable elementary particle that has a negative charge, e, of 1.602 1773 × 10–19 coulomb, mass m of 9.109 3897 × 10–31 kg, and spin ½. It is the natural unit of electric charge. Electrons are constituents of all atoms, moving around the nucleus in several possible or ‘allowed’ orbits (see also energy levels). They also exist independently. They are primarily responsible for electrical conduction in most materials (see energy bands). Electrons moving in one direction under the influence of an electric field constitute an electric current, the direction of conventional current flow being opposite to the direction of motion of the electrons. Electrons were first discovered as cathode rays by Sir J. J. Thomson in 1897.
Electrons are liberated by various effects: in gas-discharge tubes by the ionization of gas molecules; by heating metal filaments (thermionic emission); by the action of light, ultraviolet radiation, X-rays, or gamma-rays on matter (photoelectric effect); by the application of an intense electric field at the surface of a metal (field emission); by bombardment of a surface by high-speed electrons or positive ions (secondary emission).
Electrons experience an electric force FE in the presence of an electric field E; moving electrons experience a transverse force FH in the presence of a magnetic field. These have values
where B is the magnetic flux density vector acting at right angles to v, the velocity vector. These forces are utilized in the focusing of electron beams. The electric force also accelerates the electrons.
Electrons interact with matter to produce effects dependent on the velocity of the electrons and the state of matter involved. They may undergo elastic scattering, producing deflection and localized heating due to the energy lost, or inelastic collisions, losing discrete amounts of energy and producing various phenomena: with a gas, excitation and light emission or ionization occurs; with solids and liquids, effects such as the production of X-rays, fluorescence, and secondary electron emission are observed.
An electron beam also has wavelike properties, similar to those of electromagnetic radiation, the wavelength being given by
where h is the Planck constant and mv the electron’s momentum (see also de Broglie waves).
The antiparticle of the electron is the positron, which has a positive charge and a mass equal to that of the electron.