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The first few hydrogen atom electron orbitals shown as cross-sections with color-coded probability density
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The electron (sometimes called negatron; commonly represented as e−) is a subatomic particle. In an atom the electrons surround the nucleus of protons and neutrons in an electron configuration. The word electron is a transliteration of the Greek word ηλεκτρον, which means electrum, an alloy of silver and gold.
Electrons have an electrical charge and when they move, they generate an electric current. Because the electrons of an atom determine the way in which it interacts with other atoms, they play a fundamental part in chemistry.
Electrons in practice
Classification of electrons
The electron is one of a class of subatomic particles called leptons which are believed to be fundamental particles (that is, they cannot be broken down into smaller constituent parts).
The word "particle" is somewhat misleading however, because quantum mechanics shows that electrons also behave like a wave, e.g. in the double-slit experiment; this is called wave-particle duality.
Properties and behavior of electrons
The electron has a negative electric charge of −1.6 × 10−19 coulombs, and a mass of about 9.11 × 10−31 kg (0.51 MeV), which is approximately 1⁄1836 of the mass of the proton.
The motion of the electron about the nucleus is a somewhat controversial topic. The electron does not exhibit motion in the physical sense — it does not "float"; rather, it seems to appear in and out of existence, at various points around the nucleus (of course, 90% of the time the electron can be found in its designated orbital). A simple analogy would be a firefly, in a dark room, lighting up at various points about a central light source — it can light up anywhere, but it is most likely to appear closer to the source than otherwise. However the wavefunction that describes the electron's motion is completely described by the Dirac equation. The Dirac equation can be solved exactly for the hydrogen atom and it predicts precisely the allowed energy states that an electron can have -- which have been verified experimentaly by measuring the wavelength of light that is emitted from neutral hydrogen. For more complicated atoms it is not possible to solve the Dirac equation but to date there is no experimental evidence to suggest that electrons do not obey the Dirac equation.
The electron has spin ½, which implies it is a fermion, i.e., it follows the Fermi-Dirac statistics.
While most electrons are found in atoms, others move independently in matter, or together as an electron beam in a vacuum. In some superconductors, electrons move in "Cooper pairs," in which their motion is coupled to nearby matter via lattice vibrations called phonons.
When electrons move, free of the nuclei of atoms, and there is a net flow, this flow is called electricity, or an electric current.
So-called "static electricity" is not a flow of electrons. More correctly called a "static charge", it refers to a body that 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, the object is said to be electrically "neutral".
Electrons and positrons can annihilate each other and produce a photon. Conversely, a high-energy photon can be transformed into an electron and a positron by a process called pair production.
The electron is an elementary particle – that means that it has no substructure (at least, experiments have not found any so far, and there is good reason to believe that there is not any). Hence, it is usually described as point-like, i.e. with no spatial extension. (However, if one gets very near an electron, one notices that its properties (charge and mass) seem to change. This is an effect common to all elementary particles: the particle influences the vacuum fluctuations in its vicinity, so that the properties one observes from far away are the sum of the bare properties and the vacuum effects – see renormalization.)
There is a physical constant called the classical electron radius, with a value of 2.8179 × 10−15 m. Note that this is the radius that one could infer from its charge if the physics were only described by the classical theory of electrodynamics and there were no quantum mechanics (hence, it is an outdated concept, that, however, sometimes still proves useful in calculations).
The speed of an electron in a vacuum aproaches but never reaches c, the speed of light in a vacuum. This is due to an effect of special relativity. The effects of special relativity are based on a quantity known as gamma or the Lorentz factor. Gamma is a function of v, the velocity of the particle, and c. The following is the formula for gamma:
The energy necessary to accelerate a particle is gamma minus one times the rest mass. For example, the linear accelerator at Stanford can accelerate an electron to roughly 51 GeV. This gives you a gamma of 100,000 given that the rest mass of an electron is 0.51 MeV/c² (the relativistic mass of this fast electron is 100 000 times its rest mass). Solving the equation above for the speed of the electron gives a speed of 
= 0.999 999 999 95 c. (The formula applies for large γ.)
Electrons in the universe
It is believed that the number of electrons existing in the known universe is at least 1079. This number amounts to a density of about one electron per cubic metre of space.
Based on the classical electron radius and assuming a dense sphere packing, it can be calculated that the number of electrons that would fit in the observable universe is on the order of 10130. Of course, this number is even less meaningful than the classical electron radius itself.
Electrons in everyday life
The electric currents that power domestic equipment are all caused by electrons in motion. The cathode ray tube of a television set uses an electron beam in a vacuum to generate the image on the phosphorescent screen. The quantum behavior of electrons is used in semiconductor devices such as transistors.
Electrons in industry
Electron beams are used in welding as well as lithography.
Electrons in the laboratory
Founding experiments
The quantum or discrete nature of electron's charge was observed by Robert Millikan in the Oil-drop experiment of 1909.
Measurements
The spin of an electron is observed in the Stern-Gerlach experiment.
Electric charge can be directly measured with an electrometer. Electric current can be directly measured with a galvanometer.
Use of electrons in the laboratory
Electron microscopes are used to magnify details up to 500,000 times. Quantum effects of electrons are used in Scanning tunneling microscope to study features at the atomic scale.
Electrons in theory
In quantum mechanics, the electron is described by the Dirac Equation. In the Standard Model of particle physics, it forms a doublet in SU(2) with the electron neutrino, as they interact through the weak interaction. The electron has two more massive partners, with the same charge but different masses: the muon and the tauon.
The antimatter counterpart of the electron is its antiparticle, 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, each having an energy of 0.511 MeV (511 keV). See also Electron-positron annihilation.
Electrons are also a key element in electromagnetism, an approximate theory that is adequate for macroscopic systems.
History
The electron had been posited by G. Johnstone Stoney, as a unit of charge in electrochemistry, but Thompson realised that it was also a subatomic particle.
The electron was discovered by J.J. Thomson in 1897 at the Cavendish Laboratory at Cambridge University, while studying "cathode rays". Influenced by the work of James Clerk Maxwell, and the discovery of the X-ray, he deduced that cathode rays existed and were negatively charged "particles", which he called "corpuscles".
See also
External links
References
- Brumfiel, G. (6 January 2005). Can electrons do the splits? In Nature, 433, 11.
- an article about physicist Senthil Todadri ; describes efforts to reform the current understanding of electrons
Last updated: 10-14-2005 21:35:30
