electron

Negatively-charged particle forming the outer portion of all atoms. Electrons orbit the nucleus in groupings called shells. The first shell can hold up to two electrons; the second and third shells can hold up to eight electrons each; and so on. The electron arrangement of an element is called its electronic configuration; for example, the electronic configuration of the sodium atom is Na_(2,8,1). In a neutral atom the number of electrons is equal to the number of protons in the nucleus. This electron structure is responsible for the chemical properties of the atom (see atomic structure). Electrons are a member of the class of elementary particles known as leptons.

Electrons carry a charge of 1.602177 × 10⁻¹⁹ coulombs and have a mass of 9.109 × 10⁻³¹ kg, which is 1/1836 times the mass of a proton. Energy can be used to remove electrons from their outer orbits in conducting materials and they can be made to flow through these materials under a potential difference. This flow is known as an electric current or electricity. A beam of electrons will undergo diffraction (scattering) and produce interference patterns in the same way as electromagnetic waves such as light; hence they may be regarded as waves as well as particles.