Electron, one of the elementary particles that make up an atom. An electron has a negative electrical charge. The charge is equal in strength to the positive electrical charge of a proton, one of the particles that make up the nucleus of an atom. However, the mass of an electron is much less than that of a proton.

Ordinarily, an atom has an equal number of protons and electrons. The electrons occupy the outer portion of the atom and can be pictured as moving in orbits around the nucleus. Electrons are involved in all chemical reactions, and an element's chemical properties depend on the arrangement of the electrons of the element's atoms.

The movement of many electrons in one general direction gives rise to an electric current. Metals are good conductors because they contain many electrons that are only loosely bound to atoms. These electrons can readily move through the metal.

Like other elementary particles, the electron has a particle of antimatter associated with it. The particle is called an antielectron, or positron. Its discovery in 1932 confirmed a theory predicting the existence of antimatter. Electrons themselves were discovered in 1897 by J. J. Thomson, a British physicist.