The electron is a subatomic particle, symbol e− or β−, whose
electric charge is negative one elementary charge.[8] Electrons belong
to the first generation of the lepton particle family,[9] and are
generally thought to be elementary particles because they have no
known components or substructure.[1] The electron has a mass that is
approximately 1/1836 that of the proton.[10] Quantum mechanical
properties of the electron include an intrinsic angular momentum
(spin) of a half-integer value, expressed in units of the reduced
Planck constant, ħ. As it is a fermion, no two electrons can occupy
the same quantum state, in accordance with the Pauli exclusion
principle.[9] Like all elementary particles, electrons exhibit
properties of both particles and waves: they can collide with other
particles and can be diffracted like light. The wave properties of
electrons are easier to observe with experiments than those of other
particles like neutrons and protons because electrons have a lower
mass and hence a longer de Broglie wavelength for a given energy.
Electrons play an essential role in numerous physical phenomena, such
as electricity, magnetism, chemistry and thermal conductivity, and
they also participate in gravitational, electromagnetic and weak
interactions.[11] Since an electron has charge, it has a surrounding
electric field, and if that electron is moving relative to an observer
it will generate a magnetic field. Electromagnetic fields produced
from other sources will affect the motion of an electron according to
the
Contents 1 History 1.1 Discovery
1.2 Atomic theory
1.3 Quantum mechanics
1.4
2 Characteristics 2.1 Classification 2.2 Fundamental properties 2.3 Quantum properties 2.4 Virtual particles 2.5 Interaction 2.6 Atoms and molecules 2.7 Conductivity 2.8 Motion and energy 3 Formation 4 Observation 5 Plasma applications 5.1
6 See also 7 Notes 8 References 9 External links History[edit]
See also: History of electromagnetism
The ancient Greeks noticed that amber attracted small objects when
rubbed with fur. Along with lightning, this phenomenon is one of
humanity's earliest recorded experiences with electricity.[15] In his
1600 treatise De Magnete, the English scientist William Gilbert coined
the
A beam of electrons deflected in a circle by a magnetic field[27]
The German physicist
Robert Millikan While studying naturally fluorescing minerals in 1896, the French
physicist
The
By 1914, experiments by physicists Ernest Rutherford, Henry Moseley,
In quantum mechanics, the behavior of an electron in an atom is described by an orbital, which is a probability distribution rather than an orbit. In the figure, the shading indicates the relative probability to "find" the electron, having the energy corresponding to the given quantum numbers, at that point. De Broglie's prediction of a wave nature for electrons led Erwin
Schrödinger to postulate a wave equation for electrons moving under
the influence of the nucleus in the atom. In 1926, this equation, the
Schrödinger equation, successfully described how electron waves
propagated.[55] Rather than yielding a solution that determined the
location of an electron over time, this wave equation also could be
used to predict the probability of finding an electron near a
position, especially a position near where the electron was bound in
space, for which the electron wave equations did not change in time.
This approach led to a second formulation of quantum mechanics (the
first by Heisenberg in 1925), and solutions of Schrödinger's
equation, like Heisenberg's, provided derivations of the energy states
of an electron in a hydrogen atom that were equivalent to those that
had been derived first by Bohr in 1913, and that were known to
reproduce the hydrogen spectrum.[56] Once spin and the interaction
between multiple electrons were describable, quantum mechanics made it
possible to predict the configuration of electrons in atoms with
atomic numbers greater than hydrogen.[57]
In 1928, building on Wolfgang Pauli's work,
In the
Example of an antisymmetric wave function for a quantum state of two identical fermions in a 1-dimensional box. If the particles swap position, the wave function inverts its sign. Electrons are identical particles because they cannot be distinguished from each other by their intrinsic physical properties. In quantum mechanics, this means that a pair of interacting electrons must be able to swap positions without an observable change to the state of the system. The wave function of fermions, including electrons, is antisymmetric, meaning that it changes sign when two electrons are swapped; that is, ψ(r1, r2) = −ψ(r2, r1), where the variables r1 and r2 correspond to the first and second electrons, respectively. Since the absolute value is not changed by a sign swap, this corresponds to equal probabilities. Bosons, such as the photon, have symmetric wave functions instead.[84]:162–218 In the case of antisymmetry, solutions of the wave equation for interacting electrons result in a zero probability that each pair will occupy the same location or state. This is responsible for the Pauli exclusion principle, which precludes any two electrons from occupying the same quantum state. This principle explains many of the properties of electrons. For example, it causes groups of bound electrons to occupy different orbitals in an atom, rather than all overlapping each other in the same orbit.[84]:162–218 Virtual particles[edit] Main article: Virtual particle In a simplified picture, every photon spends some time as a combination of a virtual electron plus its antiparticle, the virtual positron, which rapidly annihilate each other shortly thereafter.[85] The combination of the energy variation needed to create these particles, and the time during which they exist, fall under the threshold of detectability expressed by the Heisenberg uncertainty relation, ΔE · Δt ≥ ħ. In effect, the energy needed to create these virtual particles, ΔE, can be "borrowed" from the vacuum for a period of time, Δt, so that their product is no more than the reduced Planck constant, ħ ≈ 6984660000000000000♠6.6×10−16 eV·s. Thus, for a virtual electron, Δt is at most 6979129999999999999♠1.3×10−21 s.[86] A schematic depiction of virtual electron–positron pairs appearing at random near an electron (at lower left) While an electron–positron virtual pair is in existence, the coulomb
force from the ambient electric field surrounding an electron causes a
created positron to be attracted to the original electron, while a
created electron experiences a repulsion. This causes what is called
vacuum polarization. In effect, the vacuum behaves like a medium
having a dielectric permittivity more than unity. Thus the effective
charge of an electron is actually smaller than its true value, and the
charge decreases with increasing distance from the electron.[87][88]
This polarization was confirmed experimentally in 1997 using the
Japanese TRISTAN particle accelerator.[89] Virtual particles cause a
comparable shielding effect for the mass of the electron.[90]
The interaction with virtual particles also explains the small (about
0.1%) deviation of the intrinsic magnetic moment of the electron from
the
A particle with charge q (at left) is moving with velocity v through a magnetic field B that is oriented toward the viewer. For an electron, q is negative so it follows a curved trajectory toward the top. When an electron is moving through a magnetic field, it is subject to
the
Here,
An inelastic collision between a photon (light) and a solitary (free)
electron is called Compton scattering. This collision results in a
transfer of momentum and energy between the particles, which modifies
the wavelength of the photon by an amount called the Compton
shift.[note 7] The maximum magnitude of this wavelength shift is
h/mec, which is known as the Compton wavelength.[101] For an electron,
it has a value of 6988243000000000000♠2.43×10−12 m.[70] When
the wavelength of the light is long (for instance, the wavelength of
the visible light is 0.4–0.7 μm) the wavelength shift becomes
negligible. Such interaction between the light and free electrons is
called
Atoms and molecules[edit] Main article: Atom Probability densities for the first few hydrogen atom orbitals, seen in cross-section. The energy level of a bound electron determines the orbital it occupies, and the color reflects the probability of finding the electron at a given position. An electron can be bound to the nucleus of an atom by the attractive
A lightning discharge consists primarily of a flow of electrons.[115] The electric potential needed for lightning can be generated by a triboelectric effect.[116][117] If a body has more or fewer electrons than are required to balance the positive charge of the nuclei, then that object has a net electric charge. When there is an excess of electrons, the object is said to be negatively charged. When there are fewer electrons than the number of protons in nuclei, the object is said to be positively charged. When the number of electrons and the number of protons are equal, their charges cancel each other and the object is said to be electrically neutral. A macroscopic body can develop an electric charge through rubbing, by the triboelectric effect.[118] Independent electrons moving in vacuum are termed free electrons. Electrons in metals also behave as if they were free. In reality the particles that are commonly termed electrons in metals and other solids are quasi-electrons—quasiparticles, which have the same electrical charge, spin, and magnetic moment as real electrons but might have a different mass.[119] When free electrons—both in vacuum and metals—move, they produce a net flow of charge called an electric current, which generates a magnetic field. Likewise a current can be created by a changing magnetic field. These interactions are described mathematically by Maxwell's equations.[120] At a given temperature, each material has an electrical conductivity that determines the value of electric current when an electric potential is applied. Examples of good conductors include metals such as copper and gold, whereas glass and Teflon are poor conductors. In any dielectric material, the electrons remain bound to their respective atoms and the material behaves as an insulator. Most semiconductors have a variable level of conductivity that lies between the extremes of conduction and insulation.[121] On the other hand, metals have an electronic band structure containing partially filled electronic bands. The presence of such bands allows electrons in metals to behave as if they were free or delocalized electrons. These electrons are not associated with specific atoms, so when an electric field is applied, they are free to move like a gas (called Fermi gas)[122] through the material much like free electrons. Because of collisions between electrons and atoms, the drift velocity of electrons in a conductor is on the order of millimeters per second. However, the speed at which a change of current at one point in the material causes changes in currents in other parts of the material, the velocity of propagation, is typically about 75% of light speed.[123] This occurs because electrical signals propagate as a wave, with the velocity dependent on the dielectric constant of the material.[124] Metals make relatively good conductors of heat, primarily because the delocalized electrons are free to transport thermal energy between atoms. However, unlike electrical conductivity, the thermal conductivity of a metal is nearly independent of temperature. This is expressed mathematically by the Wiedemann–Franz law,[122] which states that the ratio of thermal conductivity to the electrical conductivity is proportional to the temperature. The thermal disorder in the metallic lattice increases the electrical resistivity of the material, producing a temperature dependence for electric current.[125] When cooled below a point called the critical temperature, materials can undergo a phase transition in which they lose all resistivity to electric current, in a process known as superconductivity. In BCS theory, this behavior is modeled by pairs of electrons entering a quantum state known as a Bose–Einstein condensate. These Cooper pairs have their motion coupled to nearby matter via lattice vibrations called phonons, thereby avoiding the collisions with atoms that normally create electrical resistance.[126] (Cooper pairs have a radius of roughly 100 nm, so they can overlap each other.)[127] However, the mechanism by which higher temperature superconductors operate remains uncertain. Electrons inside conducting solids, which are quasi-particles themselves, when tightly confined at temperatures close to absolute zero, behave as though they had split into three other quasiparticles: spinons, orbitons and holons.[128][129] The former carries spin and magnetic moment, the next carries its orbital location while the latter electrical charge. Motion and energy[edit] According to Einstein's theory of special relativity, as an electron's speed approaches the speed of light, from an observer's point of view its relativistic mass increases, thereby making it more and more difficult to accelerate it from within the observer's frame of reference. The speed of an electron can approach, but never reach, the speed of light in a vacuum, c. However, when relativistic electrons—that is, electrons moving at a speed close to c—are injected into a dielectric medium such as water, where the local speed of light is significantly less than c, the electrons temporarily travel faster than light in the medium. As they interact with the medium, they generate a faint light called Cherenkov radiation.[130]
The effects of special relativity are based on a quantity known as the Lorentz factor, defined as γ = 1 / 1 − v 2 / c 2 displaystyle scriptstyle gamma =1/ sqrt 1- v^ 2 / c^ 2 where v is the speed of the particle. The kinetic energy Ke of an electron moving with velocity v is: K e = ( γ − 1 ) m e c 2 , displaystyle displaystyle K_ mathrm e =(gamma -1)m_ mathrm e c^ 2 , where me is the mass of electron. For example, the Stanford linear
accelerator can accelerate an electron to roughly 51 GeV.[131]
Since an electron behaves as a wave, at a given velocity it has a
characteristic de Broglie wavelength. This is given by
λe = h/p where h is the
The
γ + γ ↔ e+ + e− An equilibrium between electrons, positrons and photons was maintained during this phase of the evolution of the Universe. After 15 seconds had passed, however, the temperature of the universe dropped below the threshold where electron-positron formation could occur. Most of the surviving electrons and positrons annihilated each other, releasing gamma radiation that briefly reheated the universe.[134] For reasons that remain uncertain, during the annihilation process there was an excess in the number of particles over antiparticles. Hence, about one electron for every billion electron-positron pairs survived. This excess matched the excess of protons over antiprotons, in a condition known as baryon asymmetry, resulting in a net charge of zero for the universe.[135][136] The surviving protons and neutrons began to participate in reactions with each other—in the process known as nucleosynthesis, forming isotopes of hydrogen and helium, with trace amounts of lithium. This process peaked after about five minutes.[137] Any leftover neutrons underwent negative beta decay with a half-life of about a thousand seconds, releasing a proton and electron in the process, n → p + e− + ν e For about the next 7005300000000000000♠300000–7013126230400000000♠400000 years, the excess electrons remained too energetic to bind with atomic nuclei.[138] What followed is a period known as recombination, when neutral atoms were formed and the expanding universe became transparent to radiation.[139] Roughly one million years after the big bang, the first generation of stars began to form.[139] Within a star, stellar nucleosynthesis results in the production of positrons from the fusion of atomic nuclei. These antimatter particles immediately annihilate with electrons, releasing gamma rays. The net result is a steady reduction in the number of electrons, and a matching increase in the number of neutrons. However, the process of stellar evolution can result in the synthesis of radioactive isotopes. Selected isotopes can subsequently undergo negative beta decay, emitting an electron and antineutrino from the nucleus.[140] An example is the cobalt-60 (60Co) isotope, which decays to form nickel-60 (60Ni).[141] An extended air shower generated by an energetic cosmic ray striking the Earth's atmosphere At the end of its lifetime, a star with more than about 20 solar
masses can undergo gravitational collapse to form a black hole.[142]
According to classical physics, these massive stellar objects exert a
gravitational attraction that is strong enough to prevent anything,
even electromagnetic radiation, from escaping past the Schwarzschild
radius. However, quantum mechanical effects are believed to
potentially allow the emission of
π− → μ− + ν μ A muon, in turn, can decay to form an electron or positron.[147] μ− → e− + ν e + ν μ Observation[edit] Aurorae are mostly caused by energetic electrons precipitating into the atmosphere.[148] Remote observation of electrons requires detection of their radiated
energy. For example, in high-energy environments such as the corona of
a star, free electrons form a plasma that radiates energy due to
During a
Anyon
Electride
Notes[edit] ^ The fractional version's denominator is the inverse of the decimal value (along with its relative standard uncertainty of 6987420000000000000♠4.2×10−13 u). ^ The electron's charge is the negative of elementary charge, which has a positive value for the proton. ^ This magnitude is obtained from the spin quantum number as S = s ( s + 1 ) ⋅ h 2 π = 3 2 ℏ displaystyle begin alignedat 2 S&= sqrt s(s+1) cdot frac h 2pi \&= frac sqrt 3 2 hbar \end alignedat for quantum number s = 1/2. See: Gupta, M.C. (2001). Atomic and Molecular Spectroscopy. New Age Publishers. p. 81. ISBN 81-224-1300-5. ^ Bohr magneton: μ B = e ℏ 2 m e . displaystyle textstyle mu _ mathrm B = frac ehbar 2m_ mathrm e . ^ The classical electron radius is derived as follows. Assume that the electron's charge is spread uniformly throughout a spherical volume. Since one part of the sphere would repel the other parts, the sphere contains electrostatic potential energy. This energy is assumed to equal the electron's rest energy, defined by special relativity (E = mc2). From electrostatics theory, the potential energy of a sphere with radius r and charge e is given by: E p = e 2 8 π ε 0 r , displaystyle E_ mathrm p = frac e^ 2 8pi varepsilon _ 0 r , where ε0 is the vacuum permittivity. For an electron with rest mass m0, the rest energy is equal to: E p = m 0 c 2 , displaystyle textstyle E_ mathrm p =m_ 0 c^ 2 , where c is the speed of light in a vacuum. Setting them equal and
solving for r gives the classical electron radius.
See: Haken, H.; Wolf, H.C.; Brewer, W.D. (2005). The
Δ λ = h m e c ( 1 − cos θ ) , displaystyle textstyle Delta lambda = frac h m_ mathrm e c (1-cos theta ), where c is the speed of light in a vacuum and me is the electron mass. See Zombeck (2007: 393, 396). ^ The polarization of an electron beam means that the spins of all electrons point into one direction. In other words, the projections of the spins of all electrons onto their momentum vector have the same sign. References[edit] ^ a b c Eichten, E.J.; Peskin, M.E.; Peskin, M. (1983). "New Tests for
Individual physical constants from the CODATA are available at: "The NIST Reference on Constants, Units and Uncertainty". National Institute of Standards and Technology. Retrieved 2009-01-15. ^ Zombeck, M.V. (2007). Handbook of Space Astronomy and Astrophysics
(3rd ed.). Cambridge University Press. p. 14.
ISBN 0-521-78242-2.
^ Murphy, M.T.; et al. (2008). "Strong Limit on a Variable
Proton-to-
External links[edit] Wikiquote has quotations related to: Electron
Wikimedia Commons has media related to Electrons. "The Discovery of the Electron". American Institute of Physics, Center
for History of Physics.
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