The electron, symbol e⁻, is an elementary particle carrying negative electric charge. It belongs to the lepton family in the Standard Model of particle physics and is a constituent of every neutral atom. Electrons govern much of the behavior of ordinary matter, including chemical bonding and electrical conduction. Unlike composite particles such as protons, electrons are treated as point-like particles without internal constituents in the Standard Model. (home.web.cern.ch)
Fundamental properties
Every electron has the same intrinsic charge and rest mass. Its charge is −e, where the elementary charge e is exactly 1.602176634 × 10⁻¹⁹ coulombs in the International System of Units. The negative sign distinguishes its charge from that of a proton. The 2022 CODATA recommended electron rest mass is approximately 9.1093837139 × 10⁻³¹ kilograms; unlike the elementary charge, this value is experimentally determined and has a measurement uncertainty. (physics.nist.gov)
An electron has intrinsic spin angular momentum with spin quantum number 1/2, making it a fermion. Spin is a quantum property, not the literal rotation of a small material sphere. Along a chosen measurement axis, its spin projection has either of two values, +ℏ/2 or −ℏ/2. Electrons therefore obey the Pauli exclusion principle: two identical electrons cannot occupy the same complete quantum state. This restriction is fundamental to the arrangement of electrons in atoms. (openstax.org)
The electron participates in electromagnetic and weak interactions, but not directly in the strong interaction. Its antiparticle, the positron, has the same mass and opposite electric charge. Electron–positron annihilation can produce photons; at low energies, a common channel produces two photons while conserving energy and momentum. (home.web.cern.ch)
Discovery and experimental development
In 1897, J. J. Thomson investigated cathode rays, streams produced in evacuated electrical-discharge tubes. Their deflection by electric and magnetic fields allowed him to determine their charge-to-mass ratio and identify particles much lighter than atoms. His announcement on April 30, 1897, established the electron as a constituent of matter and demonstrated that atoms were not indivisible. Thomson initially called these particles “corpuscles.” (nobelprize.org)
Robert A. Millikan subsequently measured the elementary charge through experiments involving charged droplets. By balancing electrical and gravitational effects, he found that droplet charges occurred in discrete multiples of a common unit. Combining charge measurements with the charge-to-mass ratio enabled determination of the electron’s mass. Millikan received the 1923 Nobel Prize in Physics for work on the elementary charge and the photoelectric effect. (nobelprize.org)
Electron diffraction experiments later demonstrated that electrons also exhibit wave behavior. This evidence helped establish the quantum description of matter, replacing a purely classical picture of electrons as miniature charged objects following definite trajectories. (openstax.org)
Quantum behavior
In quantum mechanics, an electron’s state is described by a wave function. For an appropriate single-particle position description, the squared magnitude of this function gives the probability density for detecting the electron. An atomic orbital is a spatial wave function, not a planetary-style path around the nucleus. Orbitals describe the distributions associated with allowed electronic states. (openstax.org)
The electron’s wavelength is related to its momentum by λ = h/p, where h is the Planck constant. Consequently, electrons can produce diffraction and interference patterns. Even when electrons pass individually through a suitable double-slit apparatus, many separate detections gradually build an interference pattern. Individual localized detections and the wave-like statistical pattern are complementary features of the same quantum behavior. (openstax.org)
The relativistic theory quantum electrodynamics describes interactions between charged particles and photons. Particularly sensitive tests involve the electron’s magnetic properties, including its anomalous magnetic moment. Comparisons between precise measurements and theoretical predictions also help determine the fine-structure constant, which characterizes electromagnetic interaction strength. (nist.gov)
Atomic structure and chemical bonding
Electrons occupy states around the positively charged atomic nucleus. In a neutral atom, their number equals the number of protons. Losing electrons produces a positively charged ion, while gaining electrons produces a negatively charged ion. The organization of electronic states, especially those involved in bonding, accounts for recurring patterns in the periodic table. (openstax.org)
Electrons are central to the chemical bonds joining atoms. Covalent bonding involves shared electron density, often represented as shared electron pairs. Ionic bonding is the electrostatic attraction between oppositely charged ions, commonly formed through electron loss and gain. These descriptions connect microscopic electronic structure with the composition and properties of substances. (openstax.org)
Conduction and scientific applications
In metals, mobile electrons carry electrical current. Their average drift under an applied electric field is opposite to conventional current, whose direction is defined for positive charge. Drift speed is distinct from the much greater characteristic speeds of individual electrons within the material. (openstax.org)
In solids, allowed electron energies form bands separated by forbidden gaps. Band occupancy and gap size help explain differences between conductors, insulators, and semiconductors. Semiconductor conduction can involve both electrons and holes—the latter representing missing electrons in otherwise occupied states rather than positrons. (openstax.org)
Electron beams also provide tools for examining matter. An electron microscope uses accelerated electrons and electromagnetic lenses to form images at scales inaccessible to ordinary optical microscopy. The short wavelengths attainable by energetic electrons enable high resolution, while their diffraction by crystalline specimens supplies information about atomic arrangements. (openstax.org)