Electron, a type of elementary particle that, along with protons and neutrons, makes up atoms and molecules. Electrons play a role in a wide variety of phenomena. The flow of an electric current in a metallic conductor is caused by the drifting of free electrons in the conductor. Heat conduction in a metal is also primarily a phenomenon of electron activity. In vacuum tubes a heated cathode emits a stream of electrons that can be used to amplify or rectify an electric current (see Rectification). If such a stream is focused into a well-defined beam, it is called a cathode-ray beam (see Cathode Ray Tube). Cathode rays directed against suitable targets produce X-rays; directed against the fluorescent screen of a television tube, they produce visible images. The negatively charged beta particles emitted by some radioactive substances are electrons. See Radioactivity; Electronics; Particle Accelerators.

Electrons have a rest mass of 9.109 x 10^-31 kg and a negative electrical charge of 1.602 x 10^-19 coulombs (see Electrical Units). Electrons are classified as fermions because they have half-integral spin; spin is a quantum-mechanical property of subatomic particles that indicates the particle's angular momentum. The antimatter counterpart of the electron is the positron.