Editing Electron configuration (section)

== History ==
[[Irving Langmuir]] was the first to propose in his 1919 article "The Arrangement of Electrons in Atoms and Molecules" in which, building on [[Gilbert N. Lewis]]'s [[cubical atom]] theory and [[Walther Kossel]]'s chemical bonding theory, he outlined his "concentric theory of atomic structure".<ref>{{cite journal |last1=Langmuir |first1=Irving |author1-link=Irving Langmuir |date=June 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 }}</ref> Langmuir had developed his work on electron atomic structure from other chemists as is shown in the development of the [[History of the periodic table]] and the [[Octet rule]].
[[Niels Bohr]] (1923) incorporated Langmuir's model that the [[Periodic table|periodicity]] in the properties of the elements might be explained by the electronic structure of the atom.<ref name="Bohr">{{cite journal | last = Bohr | first = Niels | s2cid = 123582460 | author-link = Niels Bohr | title = Über die Anwendung der Quantumtheorie auf den Atombau. I | journal = Zeitschrift für Physik| year = 1923 | volume = 13 | issue = 1 | page = 117|bibcode = 1923ZPhy...13..117B |doi = 10.1007/BF01328209 }}</ref> His proposals were based on the then current [[Bohr model]] of the atom, in which the electron shells were orbits at a fixed distance from the nucleus. Bohr's original configurations would seem strange to a present-day chemist: [[sulfur]] was given as 2.4.4.6 instead of 1s<sup>2</sup>&nbsp;2s<sup>2</sup>&nbsp;2p<sup>6</sup>&nbsp;3s<sup>2</sup>&nbsp;3p<sup>4</sup> (2.8.6). Bohr used 4 and 6 following [[Alfred Werner]]'s 1893 paper. In fact, the chemists accepted the concept of atoms long before the physicists. Langmuir began his paper referenced above by saying,<blockquote>«…The problem of the structure of atoms has been attacked mainly by physicists who have given little consideration to the chemical properties which must ultimately be explained by a theory of atomic structure. The vast store of knowledge of chemical properties and relationships, such as is summarized by the Periodic Table, should serve as a better foundation for a theory of atomic structure than the relatively meager experimental data along purely physical lines... These electrons arrange themselves in a series of concentric shells, the first shell containing two electrons, while all other shells tend to [[Octet rule|hold eight]].…»</blockquote>The valence electrons in the atom were described by [[Richard Abegg]] in 1904.<ref>{{cite journal
| doi = 10.1002/zaac.19040390125
| volume = 39
| issue = 1
| pages = 330–380
| last = Abegg
| first = R.
| title = Die Valenz und das periodische System. Versuch einer Theorie der Molekularverbindungen
| trans-title = Valency and the periodic system. Attempt at a theory of molecular compounds
| journal = Zeitschrift für Anorganische Chemie
| year = 1904
| url = https://zenodo.org/record/1428102
}}</ref>

In 1924, [[Edmund Clifton Stoner|E.&nbsp;C.&nbsp;Stoner]] incorporated [[Arnold Sommerfeld|Sommerfeld's]] third quantum number into the description of electron shells, and correctly predicted the shell structure of sulfur to be 2.8.6.<ref>{{cite journal | doi = 10.1080/14786442408634535 | last = Stoner | first = E.C. | author-link = Edmund Clifton Stoner | title = The distribution of electrons among atomic levels | journal = Philosophical Magazine |series=6th Series| volume = 48 | year = 1924 | pages = 719–36 | issue = 286}}</ref> However neither Bohr's system nor Stoner's could correctly describe the changes in [[Emission spectrum|atomic spectra]] in a [[magnetic field]] (the [[Zeeman effect]]).

Bohr was well aware of this shortcoming (and others), and had written to his friend [[Wolfgang Pauli]] in 1923 to ask for his help in saving quantum theory (the system now known as "[[old quantum theory]]"). Pauli hypothesized successfully that the Zeeman effect can be explained as depending only on the response of the outermost (i.e., valence) electrons of the atom. Pauli was able to reproduce Stoner's shell structure, but with the correct structure of subshells, by his inclusion of a fourth quantum number and his [[Pauli exclusion principle|exclusion principle]] (1925):<ref>{{cite journal | last = Pauli | first = Wolfgang | s2cid = 122477612 | author-link=Wolfgang Pauli | title = Über den Einfluss der Geschwindigkeitsabhändigkeit der elektronmasse auf den Zeemaneffekt | journal = Zeitschrift für Physik| year = 1925 | volume = 31 | issue = 1 | pages = 373 | doi = 10.1007/BF02980592|bibcode = 1925ZPhy...31..373P }} English translation from {{cite journal | last = Scerri | first = Eric R. | url = http://www.chem.ucla.edu/dept/Faculty/scerri/pdf/BJPS.pdf | title = The Electron Configuration Model, Quantum Mechanics and Reduction | journal = The British Journal for the Philosophy of Science| year = 1991 | volume = 42 | issue = 3 | pages = 309–25 | doi = 10.1093/bjps/42.3.309}}</ref>
{{Blockquote|
It should be forbidden for more than one electron with the same value of the main quantum number ''n'' to have the same value for the other three quantum numbers ''k'' [{{mvar|l}}], ''j'' [''m<sub>{{mvar|l}}</sub>''] and ''m'' [''m<sub>s</sub>''].''
}}
The [[Schrödinger equation]], published in 1926, gave three of the four quantum numbers as a direct consequence of its solution for the hydrogen atom:{{efn|name="SchrodNote"}} this solution yields the atomic orbitals that are shown today in textbooks of chemistry (and above). The examination of atomic spectra allowed the electron configurations of atoms to be determined experimentally, and led to an empirical rule (known as Madelung's rule (1936),<ref name="Madelung">{{cite book | last = Madelung | first = Erwin | author-link = Erwin Madelung | title = Mathematische Hilfsmittel des Physikers | location = Berlin | publisher = Springer | year = 1936}}</ref> see below) for the order in which atomic orbitals are filled with electrons.

{{anchor|Madelung rule}}