Editing Electron (section)

=== Atomic theory ===
[[File:Bohr atom model (mul).svg|right|thumb|alt=Three concentric circles about a nucleus, with an electron moving from the second to the first circle and releasing a photon|The [[Bohr model|Bohr model of the atom]], showing states of an electron with energy [[Quantum number|quantized]] by the number ''n''. 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 Ludwig Hertz|Gustav Hertz]] had largely established the structure of an atom as a dense [[Atomic nucleus|nucleus]] of positive charge surrounded by lower-mass electrons.<ref name="smirnov" /> In 1913, Danish physicist [[Niels Bohr]] postulated that electrons resided in quantized energy states, with their energies determined by the angular momentum of the electron's orbit about the nucleus. The electrons could move between those states, or orbits, by the emission or absorption of photons of specific frequencies. By means of these quantized orbits, he accurately explained the [[spectral line]]s of the hydrogen atom.<ref>
{{cite web
 | last = Bohr
 | first = N.
 | year = 1922
 | title = Nobel Lecture: The Structure of the Atom
 | url = https://nobelprize.org/nobel_prizes/physics/laureates/1922/bohr-lecture.pdf
 | publisher = [[Nobel Foundation|The Nobel Foundation]]
 | access-date = 2008-12-03
 | df = dmy-all
 | archive-date = 2008-12-03
 | archive-url = https://web.archive.org/web/20081203124237/http://nobelprize.org/nobel_prizes/physics/laureates/1922/bohr-lecture.pdf
 | url-status = live
}}</ref> However, Bohr's model failed to account for the relative intensities of the spectral lines and it was unsuccessful in explaining the spectra of more complex atoms.<ref name="smirnov">
{{cite book
 | last = Smirnov
 | first = B.M.
 | year = 2003
 | title = Physics of Atoms and Ions
 | url = https://books.google.com/books?id=I1O8WYOcUscC&pg=PA14
 | pages = 14–21
 | publisher = [[Springer Science+Business Media|Springer]]
 | isbn = 978-0-387-95550-6
 | access-date = 2020-08-25
 | archive-date = 2020-05-09
 | archive-url = https://web.archive.org/web/20200509044538/https://books.google.com/books?id=I1O8WYOcUscC&pg=PA14
 | url-status = live
}}</ref>

Chemical bonds between atoms were explained by [[Gilbert N. Lewis|Gilbert Newton Lewis]], who in 1916 proposed that a [[covalent bond]] between two atoms is maintained by a pair of electrons shared between them.<ref>
{{cite journal
 | last = Lewis
 | first = G.N.
 | year = 1916
 | title = The Atom and the Molecule
 | journal = [[Journal of the American Chemical Society]]
 | volume = 38
 | issue = 4
 | pages = 762–786
 | doi = 10.1021/ja02261a002
 | bibcode = 1916JAChS..38..762L
 | s2cid = 95865413
 | url = https://zenodo.org/record/1429068
 | access-date = 2019-08-25
 | archive-date = 2019-08-25
 | archive-url = https://web.archive.org/web/20190825132554/https://zenodo.org/record/1429068/files/article.pdf
 | url-status = live
}}</ref> Later, in 1927, [[Walter Heitler]] and [[Fritz London]] gave the full explanation of the electron-pair formation and chemical bonding in terms of [[quantum mechanics]].<ref name=Arabatzis>
{{cite journal
 | last1 = Arabatzis | first1 = T.
 | last2 = Gavroglu | first2 = K.
 | year = 1997
 | title = The chemists' electron
 | journal = [[European Journal of Physics]]
 | volume = 18 | pages = 150–163
 | doi = 10.1088/0143-0807/18/3/005
 | bibcode = 1997EJPh...18..150A
 | issue = 3 | s2cid = 56117976
 | url = https://pdfs.semanticscholar.org/3804/783ac9fc011aeae884a3d370a474cbfdd46f.pdf
 | archive-url = https://web.archive.org/web/20200605041731/https://pdfs.semanticscholar.org/3804/783ac9fc011aeae884a3d370a474cbfdd46f.pdf
 | url-status = dead
 | archive-date = 2020-06-05
}}</ref> In 1919, the American chemist [[Irving Langmuir]] elaborated on the Lewis's static model of the atom and suggested that all electrons were distributed in successive "concentric (nearly) spherical shells, all of equal thickness".<ref>
{{cite journal
 | last = Langmuir
 | first = I.
 | year = 1919
 | title = The Arrangement of Electrons in Atoms and Molecules
 | journal = [[Journal of the American Chemical Society]]
 | volume = 41
 | issue = 6
 | pages = 868–934
 | doi = 10.1021/ja02227a002
 | bibcode = 1919JAChS..41..868L
 | url = https://zenodo.org/record/1429026
 | access-date = 2019-06-21
 | archive-date = 2021-01-26
 | archive-url = https://web.archive.org/web/20210126003324/https://zenodo.org/record/1429026
 | url-status = live
}}</ref> In turn, he divided the shells into a number of cells each of which contained one pair of electrons. With this model Langmuir was able to qualitatively explain the [[chemical property|chemical properties]] of all elements in the periodic table,<ref name=Arabatzis /> which were known to largely repeat themselves according to the [[Periodic table|periodic law]].<ref>
{{cite book
 | last = Scerri | first = E.R.
 | year = 2007
 | title = The Periodic Table
 | url = https://archive.org/details/periodictableits0000scer/page/205
 | url-access = registration | pages=205–226
 | publisher = Oxford University Press
 | isbn = 978-0-19-530573-9
}}</ref>

In 1924, Austrian physicist [[Wolfgang Pauli]] observed that the shell-like structure of the atom could be explained by a set of four parameters that defined every quantum energy state, as long as each state was occupied 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]].<ref>
{{cite book
 | last = Massimi
 | first = M.
 | year = 2005
 | title = Pauli's Exclusion Principle, The Origin and Validation of a Scientific Principle
 | url = https://books.google.com/books?id=YS91Gsbd13cC&pg=PA7
 | pages = 7–8
 | publisher = Cambridge University Press
 | isbn = 978-0-521-83911-2
 | access-date = 2020-08-25
 | archive-date = 2022-02-04
 | archive-url = https://web.archive.org/web/20220204071142/https://books.google.com/books?id=YS91Gsbd13cC&pg=PA7
 | url-status = live
}}</ref> The physical mechanism to explain the fourth parameter, which had two distinct possible values, was provided by the Dutch physicists [[Samuel Goudsmit]] and [[George Uhlenbeck]].  In 1925, they suggested that an electron, in addition to the angular momentum of its orbit, possesses an intrinsic angular momentum and [[magnetic moment|magnetic dipole moment]].<ref name="smirnov" /><ref>
{{cite journal
 | last1 = Uhlenbeck | first1 = G.E.
 | last2 = Goudsmith | first2 = S.
 | year = 1925
 | title = Ersetzung der Hypothese vom unmechanischen Zwang durch eine Forderung bezüglich des inneren Verhaltens jedes einzelnen Elektrons
 | journal = [[Naturwissenschaften|Die Naturwissenschaften]]
 | volume = 13 | issue = 47
 | bibcode = 1925NW.....13..953E
 |doi = 10.1007/BF01558878
 | pages = 953–954 | s2cid = 32211960
 |language=de
}}</ref> This is analogous to the rotation of the Earth on its axis as it orbits the Sun. The intrinsic angular momentum became known as [[Spin (physics)|spin]], and explained the previously mysterious splitting of spectral lines observed with a high-resolution [[Spectrometer|spectrograph]]; this phenomenon is known as [[fine structure]] splitting.<ref>
{{cite journal
 | last = Pauli | first = W.
 | year = 1923
 | title = Über die Gesetzmäßigkeiten des anomalen Zeemaneffektes
 | journal = [[European Physical Journal|Zeitschrift für Physik]]
 | volume = 16 | issue = 1 | pages = 155–164
 | bibcode = 1923ZPhy...16..155P
 | doi = 10.1007/BF01327386
 | s2cid = 122256737
 |language=de
}}</ref>