Electron

Theoretical estimates of the electron density for the first few hydrogen atom electron orbitals shown as cross-sections with color-coded probability density
Composition	Elementary particle
Family	Fermion
Group	Lepton
Generation	First
Interaction	Gravity, Electromagnetic, Weak
Antiparticle	Positron
Theorized	G. Johnstone Stoney (1874)
Discovered	J.J. Thomson (1897)
Symbol	e−, β−
Mass	9.10938215(45) × 10-31 kg[1] 5.48579909(27) × 10–4 u 1⁄1822.8884843(11) u 0.510998918(44) MeV/c2
Electric charge	–1.602176487(40) × 10–19 C[2]
Magnetic moment	1.0011596521859(38) μB
Spin	½
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The electron is a fundamental subatomic particle that carries a negative electric charge. It is a spin ½ lepton that participates in electromagnetic interactions, and its mass is approximately 1 / 1836 of that of the proton. Together with atomic nuclei, which consist of protons and neutrons, electrons make up atoms. The electron(s) interaction with electron(s) of adjacent nuclei is the main cause of chemical bonding.

History

Main article: History of electromagnetism

As early as 1838–51, the British natural philosopher Richard Laming conceived the idea that an atom is composed of a core of matter surrounded by sub-atomic particles that had unit electrical charges. Beginning in 1846, German physicist William Weber theorized that electricity was composed of positively and negatively charged fluids, and their interaction was govered by the inverse square law. After studying the phenomenon of electrolysis, in 1874 the Anglo-Irish physicist G. Johnstone Stoney suggested that there existed a "single definite quantity of electrity." He was able to estimate the value of the charge e of a monovalent ion by means of Faraday's laws of electrolysis.^[3] However, Stoney believed these charges were permanently attached to atoms and could not be removed. In 1881, German physicist Hermann von Helmholtz argued that both positive and negative charges were divided into elementary parts, each of which "behaves like atoms of electricity".^[4]

In 1894, Stoney coined the term electron to represent these elementary charges.

In this paper an estimate was made of the actual amount of this most remarkable fundamental unit of electricity, for which I have since ventured to suggest the name electron.

—Stoney, George Johnstone (October 1894). "Of the "Electron," or Atom of Electricity". Philosophical Magazine 38 (5): 418–420. 

The English language name electron is a combination of the word electric and the suffix -on, with the latter now used to designate a subatomic particle.^[5] Both electric and electricity are derived from the Latin ēlectrum, which in turn came from the Greek word ēlektron (ήλεκτρον) for amber; a gemstone that is formed from the hardened sap of trees. The ancient Greeks noticed that amber, when rubbed with fur, attracted small objects. This phenomenon was humankind's earliest experience with electricity, outside of lightning.^[6][7]

Lateral view of a Crookes tube, with the cathode at left. The profile of the cross-shaped anode is projected against the tube face at right by the beam of particles.

Progress in the study of electrons began to occur once a cathode ray tube was developed that had a high vacuum within its interior. Once he had accomplished during the 1870s,^[8] English chemist and physicist Sir William Crookes was able to show that the luminescence rays appearing within the tube carried energy and moved from the cathode to the anode. Further, by applying a magnetic field, he was able to deflect the rays, thereby demonstrating that the beam behaved as though it were negatively charged.^[9] In 1879, he proposed that these properties could be explained by what he termed 'radiant matter'. He suggested that this was a fourth state of matter, consisting of negatively charged molecules that were being projected with high velocity from the cathode.^[10]

The German-born British physicist Arthur Schuster expanded upon Crookes' experiments by placing metal plates in parallel to the cathode rays and applying an electrical potential between the plates. The resulting field deflected the rays toward the positive plate, providing further evidence that the rays carried negative charge. By measuring the amount of deflection for a given level of current, in 1890 Schuster was able to estimate the charge-to-mass ratio of the ray components. However, this produced such an unexpectedly large value that little credence was given to his calculations at the time.^[9]

In 1896, British physicist J.J. Thomson, with his collegues John S. Townsend and H. A. Wilson,^[11] performed experiments indicating that cathode rays really were unique particles, rather than waves, atoms or molecules as was believed earlier. Thomson made good estimates of both the charge e and the mass m, finding that cathode ray particles, which he called "corpuscles", had perhaps one thousandth of the mass of the least massive ion known (hydrogen). He also showed that their charge to mass ratio, e/m, was independent of cathode material. He further showed that the negatively charged particles produced by radioactive materials, by heated materials, and by illuminated materials, were universal.^[12] The name electron was again proposed for these particles by the Irish physicist George F. Fitzgerald, and it has since gained universal acceptance.^[9]

While studying naturally fluorescing minerals in 1896, French physicist Henri Becquerel discovered that they emitted radiation without any exposure to an external energy source. These radioactive materials became the subject of much interest by scientists, including New Zealand physicist Ernest Rutherford who discovered they emitted particles. He designated these particles alpha and beta, based on their ability to penetrate matter.^[13] In 1900, Becquerel showed that the beta rays emitted by radium could be deflected by an electrical field, and that their mass-to-charge ratio was the same as for cathode rays.^[14] This evidence strengthened the view that electrons existed as components of atoms.^[15][16]

The electron's charge was more carefully measured by American physicist R. A. Millikan in his oil-drop experiment of 1909. This experiment used an electrical field to prevent a charged droplet of oil from falling as a result of gravity. This device could measure the electrical charge from as few as 1–150 ions to within an error margin of 0.3%. Comparable experiments had been made earlier by Thomson's team, using a clouds of charged water droplets generated with electrolysis.^[11] However, oil drops, not subject to evaporation, were more stable than water drops and more suited to precise experimentation over longer periods of time.^[17]

The Bohr model of the atom, showing quantized states of electron orbital energy. An electron dropping to a lower orbit emits a photon equal to the energy difference between the orbits.

By 1914, experiments by physicists Ernest Rutherford, Henry Moseley, James Franck and Gustav Hertz had largely established the structure of an atom as a dense nucleus of positive charge surrounded by lower mass electrons.^[18] In 1913, Danish physicist Niels Bohr postulated that electrons resided in quantized energy states, with the energy determined by the angular momentum of the electron's orbits about the nucleus. The electrons could move between these states, or orbits, by the emission or absorption of photons at specific frequencies. By means of these quantized orbits, he accurately explained the spectral lines of hydrogen that were formed when the gas is energized by heat or electricity. However, Bohr's model failed to account for the relative intensities of the spectral lines and it was unsuccessful in explaining the spectrum of more complex atoms.^[18]

Chemical bonds between atoms were now explained, by Gilbert Newton Lewis in 1916, as the interactions between their constituent electrons.^[19] As the chemical properties of the elements were known to largely repeat themselves according to the periodic law,^[20] in 1919 the American chemist Irving Langmuir suggested that this could be explained if the electrons in an atom were connected or clustered in some manner. Groups of electrons were thought to occupy a set of electron shells about the nucleus.^[21]

In 1924, Austrian physicist Wolfgang Pauli observed that the shell-like structure of the atom could be explained if each quantum energy state was described by a set of four parameters, as long as each state was inhabited by no more than a single electron. (This prohibition against more than one electron occupying the same quantum energy state became known as the Pauli exclusion principle.)^[22] However, what physicists lacked was a physical mechanism to explain the fourth parameter, which had two possible values. This was provided by the Dutch physicists Abraham Goudsmith and George Uhlenbeck when they suggested that an electron, in addition to the angular momentum of its orbit, could possess an intrinsic angular momentum.^[23][24] This property became known as spin, and it explained the previously mysterious splitting of spectral lines observed with a high resolution spectrograph; a phenomenon known as fine structure splitting.^[25]

During his 1924 dissertation Recherches sur la théorie des quanta, French physicist Louis de Broglie hypothesized that all matter possesses a wave–particle duality similar to photons.^[26] That is, under the appropriate conditions, electrons and other matter would show properties of either particles or waves. The wave-like nature of light occurs, for example, when light is passed through slits, resulting in interference patterns. In 1937, a similar effect was demonstrated from a beam of electrons by English physicist George Paget Thomson with a thin metal film and by American physicists Clinton Davisson and Lester Germer using a crystal of nickel.^[27]

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.

The success of de Brolie's prediction led to the publication, by Erwin Schrödinger in 1926, of the wave equation that successfully describes how electron waves propagated.^[28] Rather than yielding a solution that determines the location of an electron over time, this wave equation gives the probability of finding an electron near a position. This approach became the theory of quantum mechanics, which provided an exact derivation to the energy states of an electron in a hydrogen atom.^[29] Once the electron spin and the interaction between multiple electrons is taken into consideration, the Schroedinger wave equation successfully predicted the configuration of electrons in atoms with higher atomic numbers than hydrogen. However, for atoms with multiple electrons, the exact solution to the wave equation is much more complicated, so approximations were often necessary.^[30]

Classification

Standard model of elementary particles. The electron is at lower left.

The electron belongs to the group of subatomic particles called leptons, which are believed to be fundamental particles. Electrons have the lowest mass of any electrically charged lepton. All members of the lepton group, which include the muon and tauon, belong to the family of fermions. This family includes all elementary particles with half-odd integer spin; the electron has spin ½. Leptons differ from the other basic constituent of matter, the quarks, by their lack of strong interaction.^[31]

The antiparticle of an electron is the positron, which has the same mass and spin as the electron but a positive rather than negative charge.^[31] The discoverer of the positron, Carl D. Anderson, proposed calling standard electrons negatrons, and using electron as a generic term to describe both the positively and negatively charged variants. This usage of the term "negatron" is still occasionally encountered today, and it may also be shortened to "negaton".^[32]

Properties and behavior

In a frame of reference where a free electron has no net velocity, its rest mass is 9.11 × 10^-31 kg.^[1] On the atomic scale, this is 5.489 × 10^-4 u, where 1 u is one-twelth the mass of a neutral ¹²C atom. Based on the principle of mass–energy equivalence, this mass corresponds to a rest energy of 0.511 MeV, where an eV, or electron volt, is defined as the energy acquired by an electron being accelerated through an electrical potential of one volt. The proton-to-electron mass ratio is about 1836.^[33] This ratio is one of the fundamental constants of physics, and the Standard Model of particle physics assumes this and other constants are unchanging. Astronomical measurements show that the mass ratio has held the same value for at least half the current age of the universe.^[34] However, the rest energy of the electron has been shown to vary by 10⁻⁶–10⁻⁹ eV because of local fluctuations of temperature and magnetic field.^[35]

Electrons have an electric charge of −1.602 × 10⁻¹⁹ C.^[2] The common electron symbol is e⁻. The electron is thought to be stable on theoretical grounds; the lowest known experimental upper bound for its mean lifetime is 4.6×10²⁶ years, with a 90% confidence interval (see Particle decay).

As with all particles, electrons can also act as waves. This is called the wave-particle duality, also known by the term complementarity coined by Niels Bohr, and can be demonstrated using the double-slit experiment. According to quantum mechanics, electrons can be represented by wavefunctions, from which a calculated probabilistic electron density can be determined. The orbital of each electron in an atom can be described by a wavefunction. Based on the Heisenberg uncertainty principle, the exact momentum and position of the actual electron cannot be simultaneously determined. This is a limitation which, in this instance, simply states that the more accurately we know a particle's position, the less accurately we can know its momentum, and vice versa.

The electron has spin ½ and is a fermion (it follows Fermi-Dirac statistics). In addition to its intrinsic angular momentum, an electron has an intrinsic magnetic moment along its spin axis.

Electrons in an atom are bound to that atom, while electrons moving freely in vacuum, space or certain media are free electrons that can be focused into an electron beam. When free electrons move, there is a net flow of charge, and this flow is called an electric current. The drift velocity of electrons in metal wires is on the order of millimetres per second. However, the speed at which a current at one point in a wire causes a current in other parts of the wire, the velocity of propagation, is typically 75% of light speed.

In some superconductors, pairs of electrons move as Cooper pairs in which their motion is coupled to nearby matter via lattice vibrations called phonons. The distance of separation between Cooper pairs is roughly 100 nm.

A body has an electric charge when that body has more or fewer electrons than are required to balance the positive charge of the nuclei. When there is an excess of electrons, the object is said to be negatively charged. When there are fewer electrons than protons, 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 phenomenon of triboelectricity.

When electrons and positrons collide, they annihilate each other and produce pairs of high-energy photons or other particles. On the other hand, high-energy photons may transform into an electron and a positron by a process called pair production, but only in the presence of a nearby charged particle, such as a nucleus.

The electron is currently described as a fundamental or elementary particle. It has no known substructure. Hence, for convenience, it is usually defined or assumed to be a point-like mathematical point charge, with no spatial extension. However, when a test particle is forced to approach an electron, we measure changes in its properties (charge and mass). This effect is common to all elementary particles. Current theory suggests that this effect is due to the influence of vacuum fluctuations in its local space, so that the properties measured from a significant distance are considered to be the sum of the bare properties and the vacuum effects (see renormalization).

The "classical electron radius" is 2.8179 × 10⁻¹⁵ m. This is the radius that is inferred from the electron's electric charge, by using the classical theory of electrodynamics alone, ignoring quantum mechanics. (In modern physics, the electron is believed to be a point particle, thus its actual radius is zero.) Classical electrodynamics (Maxwell's electrodynamics) is the older concept that is widely used for practical applications of electricity, electrical engineering, semiconductor physics, and electromagnetics. Quantum electrodynamics, on the other hand, is useful for applications involving modern particle physics and some aspects of optical, laser and quantum physics.

Based on current theory, the speed of an electron can approach, but never reach, c (the speed of light in a vacuum). This limitation is attributed to Einstein's theory of special relativity which defines the speed of light as a constant within all inertial frames. However, when relativistic electrons 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 bluish light called Cherenkov radiation.

The effects of special relativity are based on a quantity known as the Lorentz factor (γ), which is a function of the coordinate velocity of the particle (v). It is defined as:

The kinetic energy of an electron (moving with velocity v) is:

For example, the Stanford linear accelerator can accelerate an electron to roughly 51 GeV [1]. This gives a gamma of 100,000, since the mass of an electron is 0.51 MeV/c² (the relativistic momentum of this electron is 100,000 times the classical momentum of an electron at the same speed). Solving the equation above for the speed of the electron (and using an approximation for large γ) gives:

The de Broglie wavelength of a particle is λ=h/p where h is Planck's constant and p is momentum. At low (e.g photoelectron) energies this determines the size of atoms, and at high (e.g. electron microscope) energies this makes the Bragg angles for electron diffraction (co-discovered by J. J. Thomson's son G. P. Thomson) well under one degree. Since momentum is mass times proper-velocity w=γv, we have

For the 51 GeV electron above, proper-velocity is approximately γc, making the wavelength of those electrons small enough to explore structures well below the size of an atomic nucleus.

Production

The big bang theory is the current-accepted scientific theory to explain the early stages in the evolution of the Universe. For the first millisecond of the big bang, the temperatures were over 10 billion K and photons had mean energies over a million electron volts. These photons were sufficiently energetic that they could react with each other to form pairs of electrons and positrons,

where γ is a photon, e⁺ is a positron and e^- is an electron. Likewise, positron-electron pairs annihilated each other, emitting photons of gamma rays with energies of 511 keV. An equilibrium between electrons, positrons and protons was maintained during this creation and destruction cycle. 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.^[36]

For reasons that remain uncertain, there was a slight excess in the number of electrons over positrons; a problem known as baryon asymmetry.^[37] Hence a few electrons survived the annihilation process. This excess also matched the excess of protons over anti-protons, resulting in a net charge of zero for the universe. The surviving protons and neutrons begin to undergo nucleosynthesis, forming isotopes of hydrogen and helium, with trace amounts of lithium. This process peaked after a few hundred seconds, and any leftover neutrons thereafter underwent negative beta decay with a half-life of about a thousand seconds, releasing a proton and electron in the process,

where n is a neutron, p is a proton, e^- is an electron and is an electron antineutrino. For the next million years, the excess electrons remained too energetic to bind with atomic nuclei.^[38] Once atoms were formed, the universe became transparent to radiation and it continued to cool and expand.

The concentrations of mass in the universe allow stars to form. 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 also result in the synthesis of radioactive isotopes. Some of these isotopes can subsequently undergo negative beta decay, emitting an electron and antineutrino from the nucleus.^[39] An example is the cobalt-60 (⁶⁰Co) isotope, which decays to form nickel-60 (⁶⁰Ni).^[40]

Cosmic rays are particles travelling through space with high energies. Energy events as high as 3.0 × 10²⁰ eV have been recorded.^[41] When these particles collide with nucleons in the Earth's atmosphere, a shower of particles is generated, including pions.^[42] More than half of the cosmic radiation observed from the Earth's surface consists of muons. This particle is a lepton which is produced in the upper atmosphere by the decay of pions. Muons in turn can decay to form an electron or positron by means of the weak force. Thus, for the negatively charged pion π ⁻ ,^[43]

where μ ⁻ is a muon, ν_μ is a muon neutrino and is an electron antineutrino.

Visualisation

The first video images of an electron were captured by a team at Lund University in Sweden in February 2008. To capture this event, the scientists used extremely short flashes of light. To produce this light, newly developed technology for generating short pulses from intense laser light, called attosecond pulses, allowed the team at the university’s Faculty of Engineering to capture the electron's motion for the first time.

"It takes about 150 attoseconds for an electron to circle the nucleus of an atom. An attosecond is related to a second as a second is related to the age of the universe," explained Johan Mauritsson, an assistant professor in atomic physics at the Faculty of Engineering, Lund University.^[44][45]

The distribution of the electrons in the reciprocal space of solids can be visualized by angle resolved photoemission spectroscopy.

Electrons in chemistry

In 1913, Niels Bohr showed that electrons are the actual foundation of the periodic table of chemical elements, and, in 1916, Gilbert Newton Lewis explained the chemical bonding of elements by electronic interactions. From these discoveries it has become clear that electrons, in particular those orbiting on the outer shell of the atom, play a fundamental part in chemical structure and chemical interactions, and that these interactions form the central part of chemistry, without which it could not even exist.

In practice

In the universe

Scientists believe that the number of electrons existing in the known universe is at least 10⁷⁹. This number amounts to an average density of about one electron per cubic metre of space. Astronomers have estimated that 90% of the mass of atoms in the universe is hydrogen, which is made of one electron and one proton.

In industry

Electron beams are used in welding, lithography, scanning electron microscopes and transmission electron microscopes. LEED and RHEED are surface-imaging techniques that use electrons.

Electrons are also at the heart of cathode ray tubes, which are used extensively as display devices in laboratory instruments, computer monitors and television sets. In a photomultiplier tube, one photon strikes the photocathode, initiating an avalanche of electrons that produces a detectable current.

In the laboratory

The uniquely high charge-to-mass ratio of electrons means that they interact strongly with atoms, and are easy to accelerate and focus with electric and magnetic fields. Hence some of today's aberration-corrected transmission electron microscopes use 300keV electrons with velocities greater than the speed light travels in water (approximagely 1/2 to 2/3 of c), wavelengths below 2 picometers, transverse coherence-widths over a nanometer, and longitudinal coherence-widths 100 times that. This allows such microscopes to image scattering from individual atomic-nuclei (HAADF) as well as interference-contrast from solid-specimen exit-surface deBroglie-phase (HRTEM) with lateral point-resolutions down to 60 picometers. Magnifications approaching 100 million are needed to make the resulting image detail comfortably visible to the naked eye.

Quantum effects of electrons are also used in the scanning tunneling microscope to study features on solid surfaces with lateral-resolution at the atomic scale (around 200 picometers) and vertical-resolutions much better than that. In such microscopes, the quantum tunneling is strongly dependent on tip-specimen separation, and, precise control of the separation (vertical sensitivity) is made possible with a piezoelectric scanner.

In medicine

In radiation therapy, electron beams are used for treatment of superficial tumours.

In theory

In Dirac's model, an electron is defined to be a mathematical point, a point-like, charged "bare" particle surrounded by a sea of interacting pairs of virtual particles and antiparticles. These provide a correction of just over 0.1% to the predicted value of the electron's gyromagnetic ratio from exactly 2 (as predicted by Dirac's single-particle model). The extraordinarily precise agreement of this prediction with the experimentally determined value is viewed as one of the great achievements of modern physics.^[46]

In the Standard Model of particle physics, the electron is the first-generation charged lepton. It forms a weak isospin doublet with the electron neutrino; these two particles interact with each other through both the charged and neutral current weak interaction. The electron is very similar to the two more massive particles of higher generations, the muon and the tau lepton, which are identical in charge, spin, and interaction, but differ in mass.

The antimatter counterpart of the electron is the positron. The positron has the same amount of electrical charge as the electron, except that the charge is positive. It has the same mass and spin as the electron. When an electron and a positron meet, they may annihilate each other, giving rise to two gamma-ray photons emitted at roughly 180° to each other. If the electron and positron had negligible momentum, each gamma ray will have an energy of 0.511 MeV. See also Electron-positron annihilation.

Electrons are a key element in electromagnetism, a theory that is accurate for macroscopic systems, and for classical modelling of microscopic systems.

Notes and references

Book references

See also

• Atom
• Covalent bonding
• Electron bubble
• Elementary charge
• Exoelectron
• One-electron universe
• Pair production
• Positron
• Proton

External links

Wikisource has the text of the 1911 Encyclopædia Britannica article Electron.

• The Discovery of the Electron from the American Institute of Physics History Center
• Particle Data Group
• Eric Weisstein's World of Physics: Electron
• Researchers Catch Motion of a Single Electron on Video