- Pauli exclusion principle
The Pauli exclusion principle is a quantum mechanical principle formulated by
Wolfgang Pauliin 1925. It states that no two identical fermionsmay occupy the same quantum state"simultaneously". A more rigorous statement of this principle is that, for two identical fermions, the total wave function is anti-symmetric. For electrons in a single atom, it states that no two electrons can have the same four quantum numbers, that is, if "n", "l", and "ml" are the same, "ms" must be different such that the electrons have opposite spins.
quantum field theory, the Pauli principle follows from applying a rotation operator in imaginary time to particles of half-integer spin. It does not follow from any spin relation in nonrelativistic quantum mechanics.
The Pauli exclusion principle is one of the most important principles in
physics, primarily because the three types of particles from which ordinary matteris made— electrons, protons, and neutrons—are all subject to it; consequently, all material particles exhibit space-occupying behavior. The Pauli exclusion principle underpins many of the characteristic properties of matter from the large-scale stability of matter to the existence of the periodic table of the elements.
Particles with antisymmetric wave functions are called
fermions—and obey the Pauli exclusion principle. Apart from the familiar electron, proton and neutron, these include neutrinos and quarks (from which protons and neutrons are made), as well as some atoms like helium-3. All fermions possess "half-integer spin", meaning that they possess an intrinsic angular momentumwhose value is ( Planck's constantdivided by 2π) times a half-integer(1/2, 3/2, 5/2, etc.). In the theory of quantum mechanics, fermions are described by "antisymmetric states", which are explained in greater detail in the article on identical particles.
Particles with integer spin have a symmetric wave function and are called
bosons; in contrast to fermions, they may share the same quantum states. Examples of bosons include the photonand the W and Z bosons.
In the early 20th century, it became evident that atoms and molecules with pairs of electrons or even numbers of electrons are more stable than those with odd numbers of electrons. In the famous 1916 article " [http://dbhs.wvusd.k12.ca.us/webdocs/Chem-History/Lewis-1916/Lewis-1916.html The Atom and the Molecule] " by
Gilbert N. Lewis, for example, rule three of his six postulates of chemical behavior states that the atom tends to hold an even number of electrons in the shell and especially to hold eight electrons which are normally arranged symmetrically at the eight corners of a cube (see: cubical atom). In 1919, the American chemist Irving Langmuirsuggested that the periodic tablecould 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. [cite journal
last=Langmuir | first=Irving
title=The Arrangement of Electrons in Atoms and Molecules
journal=Journal of the American Chemical Society
year=1919 | volume=41 | issue=6 | pages=868–934
accessdate=2008-09-01 ] In 1922,
Niels Bohrupdated his model of the atom by assuming that certain numbers of electrons (for example 2, 8 and 18) corresponded to stable "closed shells".
Pauli looked for an explanation for these numbers which were at first only empirical.At the same time he was trying to explain experimental results in the
Zeeman effectin atomic spectroscopy and in ferromagnetism. He found an essential clue in a 1924 paper by E.C.Stoner which pointed out that for a given value of the principal quantum number(n), the number of energy levels of a single electron in the alkali metalspectra in an external magnetic field, where all degenerate energy levels are separated, is equal to the number of electrons in the closed shell of the rare gases for the same value of n. This led Pauli to realize that the complicated numbers of electrons in closed shells can be reduced to the simple rule "one" per state, if the electron states are defined using four quantum numbers. For this purpose he introduced a new two-valued quantum number, identified by Samuel Goudsmitand George Uhlenbeckas electron spin.
Connection to quantum state symmetry
The Pauli exclusion principle with a single-valued many-particle wavefunction is equivalent to the assumption that the wavefunction is antisymmetric. An antisymmetric two-particle state is represented as a sum of states in which one particle is in state and the other in state ::
and antisymmetry under exchange means that A(x,y) = -A(y,x). This implies that A(x,x)=0, which is Pauli exclusion. It is true in any basis, since unitary changes of basis keep antisymmetric matrices antisymmetric, although strictly speaking, the quantity A(x,y) is not a matrix but an antisymmetric rank two tensor.
Conversely, if the diagonal quantities A(x,x) are zero "in every basis", then the wavefunction component::
is necessarily antisymmetric. To prove it, consider the matrix element::
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Pauli exclusion principle — n. [after PAULI Wolfgang] the principle that no two electrons, protons, etc. in a given system can have the same set of quantum numbers and, thus, that no two can occupy the same space at the same time: see FERMION … English World dictionary
Pauli exclusion principle — Physics. See exclusion principle. [1925 30; named after W. PAULI] * * * Assertion proposed by Wolfgang Pauli that no two electrons in an atom can be in the same state or configuration at the same time. It accounts for the observed patterns of… … Universalium
Pauli exclusion principle — exclusion principle, principle put forward by the Austrian physicist Wolfgang Pauli that no two electrons in an atom can be exactly equivalent … English contemporary dictionary
Pauli exclusion principle — noun Etymology: Wolfgang Pauli Date: 1926 exclusion principle called also Pauli principle … New Collegiate Dictionary
Pauli exclusion principle — Pau′li exclu′sion prin ciple n. phs exclusion principle • Etymology: 1925–30; after W. Pauli … From formal English to slang
Pauli exclusion principle — [ paʊli] noun Physics the assertion that no two fermions can have the same quantum number. Origin 1920s: named after the American physicist Wolfgang Pauli … English new terms dictionary
Pauli exclusion principle — n. Physics the assertion that no two fermions can have the same quantum number. Etymology: W. Pauli, Austrian physicist d. 1958 … Useful english dictionary
Pauli exclusion principle — noun A principle in quantum mechanics that states that no two identical fermions may occupy the same quantum state simultaneously … Wiktionary
Exclusion principle — may refer to: * The Exclusion principle, an epistemological principle * In economics, the exclusion principle states the owner of a private good may exclude others from use unless they pay. * The Pauli exclusion principle, a quantum mechanical… … Wikipedia
exclusion principle — n. PAULI EXCLUSION PRINCIPLE … English World dictionary