Editing Pauli exclusion principle (section)

== History ==

In the early 20th century it became evident that atoms and molecules with even numbers of electrons are more [[Chemical stability#Outside chemistry|chemically stable]] than those with odd numbers of electrons. In the 1916 article "The Atom and the Molecule" by [[Gilbert N. Lewis]], for example, the third of his six postulates of chemical behavior states that the atom tends to hold an even number of electrons in any given shell, and especially to hold eight electrons, which he assumed to be typically arranged symmetrically [[Cubical atom|at the eight corners of a cube]].<ref>{{Cite web|url=http://scarc.library.oregonstate.edu/coll/pauling/bond/index.html|title=Linus Pauling and The Nature of the Chemical Bond: A Documentary History |publisher=Special Collections & Archives Research Center - Oregon State University|via=scarc.library.oregonstate.edu}}</ref> In 1919 chemist [[Irving Langmuir]] suggested that the [[periodic table]] 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 shell]]s around the nucleus.<ref>
{{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
 |url         = http://www.physics.kku.ac.th/estructure/files/Langmuir_1919_AEA.pdf
 |access-date  = 2008-09-01
 |doi         = 10.1021/ja02227a002
 |archive-url  = https://www.webcitation.org/66YZ6UWkA?url=http://www.physics.kku.ac.th/estructure/files/Langmuir_1919_AEA.pdf
 |archive-date = 2012-03-30
}}</ref> In 1922, [[Niels Bohr]] updated [[Bohr model|his model of the atom]] by assuming that certain numbers of electrons (for example 2, 8 and 18) corresponded to stable "closed shells".<ref name=Shaviv>{{cite book
 | last =Shaviv
 | first =Glora
 | title =The Life of Stars: The Controversial Inception and Emergence of the Theory of Stellar Structure 
 | publisher =Springer
 | date =2010 
 | isbn =978-3-642-02087-2 
}}</ref>{{rp|203}}

Pauli looked for an explanation for these numbers, which were at first only [[Empirical relationship|empirical]]. At the same time he was trying to explain experimental results of the [[Zeeman effect]] in atomic [[spectroscopy]] and in [[ferromagnetism]]. He found an essential clue in a 1924 paper by [[Edmund Clifton Stoner|Edmund 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 metal]] spectra in an external magnetic field, where all [[degenerate energy level]]s are separated, is equal to the number of electrons in the closed shell of the [[noble gas]]es 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 of ''one'' electron 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 Goudsmit]] and [[George Uhlenbeck]] as [[electron spin]].<ref name=Straumann>
{{cite journal
 | last =Straumann
 | first =Norbert
 | title =The Role of the Exclusion Principle for Atoms to Stars: A Historical Account
 | journal =Invited Talk at the 12th Workshop on Nuclear Astrophysics
 | date =2004
 | pages =184–196
 | arxiv =quant-ph/0403199| citeseerx =10.1.1.251.9585
 | bibcode =2004quant.ph..3199S
}}</ref><ref>{{cite journal | doi = 10.1007/BF02980631 | volume=31 | title=Über den Zusammenhang des Abschlusses der Elektronengruppen im Atom mit der Komplexstruktur der Spektren | year=1925 | journal=Zeitschrift für Physik | pages=765–783 | last1 = Pauli | first1 = W.| issue=1 | bibcode=1925ZPhy...31..765P | s2cid=122941900 }}</ref>