Electron
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Hydrogen atom electron orbitals
The first few hydrogen atom electron orbitals
shown as cross-sections with color-coded probability density
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The
electron (also 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 sometimes 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/c²), which is approximately
1⁄
1836 of the mass of the
proton.
The motion of the electron about the nucleus is a somewhat controversial topic. Electrons may only contain, in integers of
h, which is 6.63 × 10
−11 Js (this is known as Planck's constant), the amount they contain determines which shell they orbit in (k, l, m, etc.) 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. By this analogy, the electron can "appear" anywhere, and while its next location can be generalized, where it will pop up exactly can never be known. For this reason many scientists believe that the motion of the electron can never be fully understood.
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 pairs.
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 that would fit in the known
universe is 10
130 (that is, 1 followed by 130 zeros.)
It is believed that the number of electrons that exist in the known
universe is at least 10
79 (that is, 1 followed by 79 zeros.)
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
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