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elementary particleelementary particles,
the most basic physical constituents of the universe. Basic Constituents of Matter

Molecules are built up from the atom, which is the basic unit of any chemical element. The atom in turn is made from the proton, neutron, and electron.
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 carrying a unit charge of negative electricity. Ordinary electric current is the flow of electrons through a wire conductor (see electricityelectricity,
class of phenomena arising from the existence of charge. The basic unit of charge is that on the proton or electron—the proton's charge is designated as positive while the electron's is negative.
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). The electron is one of the basic constituents of matter. An atomatom
[Gr.,=uncuttable (indivisible)], basic unit of matter; more properly, the smallest unit of a chemical element having the properties of that element. Structure of the Atom
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 consists of a small, dense, positively charged nucleus surrounded by electrons that whirl about it in orbits, forming a cloud of charge. Ordinarily there are just enough negative electrons to balance the positive charge of the nucleus, and the atom is neutral. If electrons are added or removed, a net charge results, and the atom is said to be ionized (see ionion,
atom or group of atoms having a net electric charge. Positive and Negative Electric Charges

A neutral atom or group of atoms becomes an ion by gaining or losing one or more electrons or protons.
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). Atomic electrons are responsible for the chemical properties of matter (see valencevalence,
combining capacity of an atom expressed as the number of single bonds the atom can form or the number of electrons an element gives up or accepts when reacting to form a compound.
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). The name electron was first used for a unit of negative electricity by the English physicist G. J. Stoney in the late 19th cent. The actual discovery of the particle, however, was made in 1897 by J. J. Thomson, who showed that cathode rays are composed of electrons and who measured the ratio of charge to mass for the electron. In 1909, R. A. Millikan measured the charge of the electron. Combining these two results gives the mass of the electron (about 1/1,840 of the mass of the proton). Ernest Rutherford, in 1903, showed that beta rays (see radioactivityradioactivity,
spontaneous disintegration or decay of the nucleus of an atom by emission of particles, usually accompanied by electromagnetic radiation. The energy produced by radioactivity has important military and industrial applications.
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) are high-energy electrons. In 1927, Davisson and Germer, working with high-speed electron beams, discovered that electrons sometimes exhibit the wave property of diffraction. This confirmed L. V. de Broglie's hypothesis that electrons, which had previously been thought of as particles, also possess certain wave properties (see quantum theoryquantum theory,
modern physical theory concerned with the emission and absorption of energy by matter and with the motion of material particles; the quantum theory and the theory of relativity together form the theoretical basis of modern physics.
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). The wavelike properties of electrons are utilized in the electron microscopemicroscope,
optical instrument used to increase the apparent size of an object. Simple Microscopes

A magnifying glass, an ordinary double convex lens having a short focal length, is a simple microscope. The reading lens and hand lens are instruments of this type.
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 and other devices. The electron is the lightest particle having a non-zero rest mass. It belongs to the leptonlepton
[Gr.,=light (i.e., lightweight)], class of elementary particles that includes the electron and its antiparticle, the muon and its antiparticle, the tau and its antiparticle, and the neutrino and antineutrino associated with each of these particles.
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 class of particles and, together with its antiparticleantiparticle,
elementary particle corresponding to an ordinary particle such as the proton, neutron, or electron, but having the opposite electrical charge and magnetic moment.
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, the positron, and its associated neutrinoneutrino
[Ital.,=little neutral (particle)], elementary particle with no electric charge and a very small mass emitted during the decay of certain other particles. The neutrino was first postulated in 1930 by Wolfgang Pauli in order to maintain the law of conservation of energy
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 and antineutrino, constitutes a subfamily of the leptons. In any particle reaction involving any of the four members of the electron family, the total electron family number (+1 for ordinary particles, −1 for antiparticles) must be conserved (see conservation lawsconservation laws,
in physics, basic laws that together determine which processes can or cannot occur in nature; each law maintains that the total value of the quantity governed by that law, e.g., mass or energy, remains unchanged during physical processes.
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, in physics). As a consequence, an electron and a positron (total electron family number equals zero) can annihilate each other to yield two or more photons or a neutrino-antineutrino pair, but not two neutrinos (total electron family number equals two).


An elementary particle which is the negatively charged constituent of ordinary matter. The electron is the lightest known particle which possesses an electric charge. Its rest mass is me ≅ 9.1 × 10-28 g, about 1/1836 of the mass of the proton or neutron, which are, respectively, the positively charged and neutral constituents of ordinary matter. Discovered in 1895 by J. J. Thomson in the form of cathode rays, the electron was the first elementary particle to be identified. See Electric charge, Elementary particle, Nuclear structure

The charge of the electron is -e ≅ -4.8 × 10-10 esu = -1.6 × 10-19 coulomb. The sign of the electron's charge is negative by convention, and that of the equally charged proton is positive. This is a somewhat unfortunate convention, because the flow of electrons in a conductor is thus opposite to the conventional direction of the current.

Electrons are emitted in radioactivity (as beta rays) and in many other decay processes; for instance, the ultimate decay products of all mesons are electrons, neutrinos, and photons, the meson's charge being carried away by the electrons. The electron itself is completely stable. Electrons contribute the bulk to ordinary matter; the volume of an atom is nearly all occupied by the cloud of electrons surrounding the nucleus, which occupies only about 10-13 of the atom's volume. The chemical properties of ordinary matter are determined by the electron cloud. See Meson, Radioactivity

The electron obeys the Fermi-Dirac statistics, and for this reason is often called a fermion. One of the primary attributes of matter, impenetrability, results from the fact that the electron, being a fermion, obeys the Pauli exclusion principle; the world would be completely different if the lightest charged particle were a boson, that is, a particle that obeys Bose-Einstein statistics. See Bose-Einstein statistics, Exclusion principle, Fermi-Dirac statistics, Positron

Magnetic moment

The electron has magnetic properties by virtue of (1) its orbital motion about the nucleus of its parent atom and (2) its rotation about its own axis. The magnetic properties are best described through the magnetic dipole moment associated with 1 and 2. The classical analog of the orbital magnetic dipole moment is the dipole moment of a small current-carrying circuit. The electron spin magnetic dipole moment may be thought of as arising from the circulation of charge, that is, a current, about the electron axis; but a classical analog to this moment has much less meaning than that to the orbital magnetic dipole moment. The magnetic moments of the electrons in the atoms that make up a solid give rise to the bulk magnetism of the solid.


That property of an electron which gives rise to its angular momentum about an axis within the electron. Spin is one of the permanent and basic properties of the electron. Both the spin and the associated magnetic dipole moment of the electron were postulated by G. E. Uhlenbeck and S. Goudsmit in 1925 as necessary to allow the interpretation of many observed effects, among them the so-called anomalous Zeeman effect, the existence of doublets (pairs of closely spaced lines) in the spectra of the alkali atoms, and certain features of x-ray spectra. See Spin (quantum mechanics)

The spin quantum number is s, where s is always ½. This means that the component of spin angular momentum along a preferred direction, such as the direction of a magnetic field, is ± ½ℏ, where ℏ is Planck's constant h divided by 2π. The spin angular momentum of the electron is not to be confused with the orbital angular momentum of the electron associated with its motion about the nucleus. In the latter case the maximum component of angular momentum along a preferred direction is lℏ, where l is the angular momentum quantum number and may be any positive integer or zero. See Quantum numbers

The electron has a magnetic dipole moment by virtue of its spin. The approximate value of the dipole moment is the Bohr magneton μ0 which is equal to eh/4πmc = 9.27 × 10-21 erg/oersted, where e is the electron charge measured in electrostatic units, m is the mass of the electron, and c is the velocity of light. (In SI units, μ0 = 9.27 × 10-24 joule/tesla.) The orbital motion of the electron also gives rise to a magnetic dipole moment μl, that is equal to μ0 when l = 1.


(i-lek -tron) Symbol: e. A stable elementary particle (a lepton) that has a negative charge of 1.6022 × 10–19 coulomb, a mass of 9.1094 × 10–28 gram, and spin ½. Although it has mass, the electron has no size: it is described as ‘pointlike’. Electrons are constituents of all atoms, moving around the central nucleus. They can also exist independently. The antiparticle of the electron is the positron, symbol e+ or ē.



(symbol e, e), the first elementary particle discovered in physics; the material carrier of the smallest mass and electric charge in nature. Electrons are constituents of atoms; the number of electrons in a neutral atom is equal to the atomic number, that is, to the number of protons in the nucleus (seeATOM).

The present values of the charge (e) and mass (me) of the electron are

e = –4.803242(14) × 10–10 cgse units

= –1.6021892(46) × 10–19coulomb

me = 0.9109534(47) × 10–27g

= 0.5110034(14) megaelectron-volt/c2

where c is the speed of light in a vacuum (the root-mean-square errors in the last significant digits are indicated in parentheses after the numerical values of the quantities). The electron has spin ½ (in units of Planck’s constant ℏ), and consequently it obeys Fermi-Dirac statistics. Its magnetic moment is μe = 1.0011596567(35)μ0, where μ0 is Bohr’s magneton. The electron is a stable particle and belongs to the lepton class. (SeeSPIN; PLANCK’S CONSTANT; FERMI-DIRAC STATISTICS; MAGNETON; and LEPTONS.)

The road to the discovery of the electron was paved by the work of many outstanding scientists. The electron was discovered in 1897 by J. J. Thomson. The word “electron,” which was originally suggested by the British scientist G. Stoney in 1891 for the charge of a univalent ion, is derived from the Greek word elektron, which means “amber.” It was agreed to designate as negative the electric charge of the electron, in conformance with the earlier convention of designating as negative the charge of electrified amber (see). The antiparticle of the electron, the positron (e+), was discovered in 1932 (seeANTI-PARTICLES).

The electron participates in the electromagnetic, weak, and gravitational interactions and exhibits a variety of properties, depending on the type of interaction. In classical electrodynamics the electron behaves like a particle whose motion obeys the Lorentz-Maxwell equations (seeLORENTZ-MAXWELL EQUATIONS). The concept of an electron’s size cannot be formulated noncontradictorily, although r0 = e2/mec2 × 10–13 cm is commonly called the classical radius of the electron. The reason for this can be understood within the framework of quantum mechanics. According to de Brogue’s hypothesis (1924), the electron, like all other material microobjects, exhibits wave as well as corpuscular properties. The length of a de Broglie wave of an electron is equal to λ = 2πℏ/mev, where v is the electron’s velocity (seeWAVE-PARTICLE DUALITY and DE BROGLIE WAVES). Accordingly, the electron, like light, may experience interference and diffraction. The wave properties of the electron were experimentally detected in 1927 by the American physicists C. Davisson and L. Germer and, independently, by the British physicist G. P. Thomson (seeDIFFRACTION OF PARTICLES).

The motion of the electron obeys the equations of quantum mechanics: the Schrödinger equation for nonrelativistic phenomena and the Dirac equation for relativistic phenomena (seeSCHRÖDINGER EQUATION and DIRAC EQUATION). Using these equations, it is possible to show that all optical, electrical, magnetic, chemical, and mechanical properties of substances are attributable to the distinctive features of the motion of electrons in atoms. The presence of spin significantly influences the nature of an electron’s motion in an atom. In particular, only by taking into account the spin of the electron within the framework of quantum mechanics was it possible to explain D. I. Mendeleev’s periodic table of the elements and the nature of the chemical bond of atoms in molecules.

The electron is a member of an extensive unified family of elementary particles and fully possesses one of the basic properties of elementary particles—interconvertibility. The electron may be produced in various reactions, of which the best known are the decay of a negatively charged muon (μ) into an electron, an electron antineutrino (v̄e), and a muon neutrino (vμ),

μ → e + e + vμ

and the beta decay of a neutron into a proton, electron, and electron antineutrino,

n → p + e+ + v̄e

The latter reaction is the source of beta rays in the radioactive decay of nuclei. Both processes are particular cases of the weak interactions (seeWEAK INTERACTION). The annihilation of an electron and a positron into two gamma quanta (e + e+ → 2γ) is an example of electromagnetic processes in which the transformations of electrons occurs.

Since the 1960’s, intensive study has been under way of the processes of the production of strongly interacting particles (hadrons) upon the collision of electrons with positrons, for example, the production of a pair of pions (seePIONS):

e + e+ → π + π+

In late 1974, a new elementary particle, the Jjψ particle, was discovered in a similar reaction) (seeRESONANCE PARTICLE and ).

The relativistic quantum theory of the electron (seeQUANTUM ELECTRODYNAMICS) is the most developed branch of quantum field theory, and surprising agreement with experiment has been achieved. For example, the calculated value of the magnetic moment of the electron,

where α ≈ 1/137.036 is the fine-structure constant, agrees with the experimental value with great accuracy (seeFINE-STRUCTURE CONSTANT). However, the theory of the electron cannot be considered complete, since it contains intrinsic logical contradictions (seeQUANTUM FIELD THEORY).


Millikan, R. Elektrony (+ i –), protony, fotony, neitrony i kosmicheskie luchi. Moscow-Leningrad, 1939. (Translated from English.)
Anderson, D. Otkrytie elektrona. Moscow, 1968. (Translated from English.)
Thomson, G. P. “Semidesiatiletnii elektron.” Uspekhi fizicheskikh nauk, 1968, vol. 94, issue 2. (Translated from English.)



A stable elementary particle which is the negatively charged constituent of ordinary matter, having a mass of about 9.11 × 10-28 gram (equivalent to 0.511 MeV), a charge of about -1.602 × 10-19 coulomb, and a spin of ½. Also known as negative electron; negatron.
Collective name for the electron, as in the first definition, and the positron.


a stable elementary particle present in all atoms, orbiting the nucleus in numbers equal to the atomic number of the element in the neutral atom; a lepton with a negative charge of 1.602 176 462 × 10--19 coulomb, a rest mass of 9.109 381 88 × 10--31 kilogram, a radius of 2.817 940 285 × 10--15 metre, and a spin of ½


A sub-atomic particle with a negative quantised charge. A flow of electrical current consists of the unidirectional (on average) movement of many electrons. The more mobile electrons are in a given material, the greater it electrical conductance (or equivalently, the lower its resistance).


An elementary particle that orbits the nucleus of an atom. There is one electron for every proton in the nucleus, which keeps the atom "electrically neutral," as electrons are considered to have a negative charge and protons a positive charge.

Electron Imbalance Creates a Charge
When the number of electrons and protons are unequal, the atom is electrically charged. If there are more electrons than protons, it is negatively charged; if fewer electrons, it is positively charged. Electrical generators strip electrons away from their atoms to provide electricity, and electrons are removed, as well as added, to enable other electrical effects (see ion and n-type silicon). See atom, quantum state, wave-particle duality and photon.
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