Editing Oxidation state (section)

== History of the oxidation state concept ==
=== Early days ===
Oxidation itself was first studied by [[Antoine Lavoisier]], who defined it as the result of reactions with [[oxygen]] (hence the name).<ref>{{cite web|url=https://www.acs.org/content/acs/en/education/whatischemistry/landmarks/lavoisier.html|title=Antoine Laurent Lavoisier The Chemical Revolution – Landmark – American Chemical Society|website=American Chemical Society|access-date=14 July 2018|archive-date=5 January 2021|archive-url=https://web.archive.org/web/20210105040544/https://www.acs.org/content/acs/en/education/whatischemistry/landmarks/lavoisier.html|url-status=live}}</ref><ref>{{cite web|url=http://chem125-oyc.webspace.yale.edu/125/history99/2Pre1800/Lavoisier/Nomenclature/Lavoisier_on_Elements.html|title=Lavoisier on Elements|website=Chem125-oyc.webspace.yale.edu|access-date=14 July 2018|archive-date=13 June 2020|archive-url=https://web.archive.org/web/20200613025757/http://chem125-oyc.webspace.yale.edu/125/history99/2Pre1800/Lavoisier/Nomenclature/Lavoisier_on_Elements.html|url-status=live}}</ref> The term has since been generalized to imply a ''formal'' loss of electrons. Oxidation states, called ''oxidation grades'' by [[Friedrich Wöhler]] in 1835,<ref>{{cite book|first=F.|last=Wöhler|title=Grundriss der Chemie: Unorganische Chemie|trans-title=Foundations of Chemistry: Inorganic Chemistry|publisher=Duncker und Humblot|location=Berlin|date=1835|page=4}}</ref> were one of the intellectual stepping stones that [[Dmitri Mendeleev]] used to derive the [[periodic table]].<ref>{{Greenwood&Earnshaw|pages=33}}</ref> [[William B. Jensen]]<ref>{{cite journal|author1-link=William B. Jensen|first=W. B.|last=Jensen|title=the origin of the oxidation-state concept|journal=J. Chem. Educ.|volume=84|issue=9|date=2007|pages=1418–1419|doi=10.1021/ed084p1418|bibcode=2007JChEd..84.1418J}}</ref> gives an overview of the history up to 1938.

=== Use in nomenclature ===
When it was realized that some metals form two different binary compounds with the same nonmetal, the two compounds were often distinguished by using the ending ''-ic'' for the higher metal oxidation state and the ending ''-ous'' for the lower. For example, FeCl<sub>3</sub> is [[ferric chloride]] and FeCl<sub>2</sub> is [[ferrous chloride]]. This system is not very satisfactory (although sometimes still used) because different metals have different oxidation states which have to be learned: ferric and ferrous are +3 and +2 respectively, but cupric and cuprous are +2 and +1, and stannic and stannous are +4 and +2. Also, there was no allowance for metals with more than two oxidation states, such as [[vanadium]] with oxidation states +2, +3, +4, and +5.<ref name=Petrucci />{{rp|84}}

This system has been largely replaced by one suggested by [[Alfred Stock]] in 1919<ref>{{cite journal|first=A.|last=Stock|title=Einige Nomenklaturfragen der anorganischen Chemie|trans-title=Some nomenclature issues of inorganic chemistry|journal=Angew. Chem.|volume=32|issue=98|date=1919|pages=373–374|doi=10.1002/ange.19190329802|bibcode=1919AngCh..32..373S|url=https://zenodo.org/record/1424478|access-date=2019-07-01|archive-date=2020-08-06|archive-url=https://web.archive.org/web/20200806044615/https://zenodo.org/record/1424478|url-status=live}}</ref> and adopted<ref name="1940IUPACinorgnom">{{cite journal|first1=W. P.|last1=Jorissen|first2=H.|last2=Bassett|first3=A.|last3=Damiens|first4=F.|last4=Fichter|first5=H.|last5=Rémy|title=Rules for naming inorganic compounds|journal=J. Am. Chem. Soc.|volume=63|date=1941|pages=889–897|doi=10.1021/ja01849a001}}</ref> by [[IUPAC]] in 1940. Thus, FeCl<sub>2</sub> was written as [[iron(II) chloride]] rather than ferrous chloride. The Roman numeral II at the central atom came to be called the "[[Stock nomenclature|Stock number]]" (now an obsolete term), and its value was obtained as a charge at the central atom after removing its ligands along with the [[electron pair]]s they shared with it.<ref name="RedBook2005" />{{rp|147}}

=== Development towards the current concept ===
The term "oxidation state" in English chemical literature was popularized by [[Wendell Mitchell Latimer]] in his 1938 book about electrochemical potentials.<ref>{{cite book|first=W. M.|last=Latimer|title=The Oxidation States of the Elements and their Potentials in Aqueous Solutions|edition=1st|publisher=Prentice-Hall|date=1938}}</ref> He used it for the value (synonymous with the German term ''Wertigkeit'') previously termed "valence", "polar valence" or "polar number"<ref>{{cite journal|first1=W. C.|last1=Bray|first2=G. E. K.|last2=Branch|title=Valence and tautomerism|journal=J. Am. Chem. Soc.|volume=35|issue=10|date=1913|pages=1440–1447|doi=10.1021/ja02199a003|bibcode=1913JAChS..35.1440B |url=https://zenodo.org/record/1428999|access-date=2019-09-16|archive-date=2021-02-09|archive-url=https://web.archive.org/web/20210209031533/https://zenodo.org/record/1428999|url-status=live}}</ref> in English, or "oxidation stage" or indeed<ref>{{cite journal|first1=A. A.|last1=Noyes|first2=K. S.|last2=Pitzer|first3=C. L.|last3=Dunn|title=Argentic salts in acid solution, I. The oxidation and reduction reactions|journal=J. Am. Chem. Soc.|volume=57|issue=7|date=1935|pages=1221–1229|doi=10.1021/ja01310a018|bibcode=1935JAChS..57.1221N }}</ref><ref>{{cite journal|first1=A. A.|last1=Noyes|first2=K. S.|last2=Pitzer|first3=C. L.|last3=Dunn|title=Argentic salts in acid solution, II. The oxidation state of argentic salts|journal=J. Am. Chem. Soc.|volume=57|issue=7|date=1935|pages=1229–1237|doi=10.1021/ja01310a019|bibcode=1935JAChS..57.1229N }}</ref> the "state of oxidation". Since 1938, the term "oxidation state" has been connected with [[electrochemical potential]]s and electrons exchanged in [[redox couple]]s participating in redox reactions. By 1948, IUPAC used the 1940 nomenclature rules with the term "oxidation state",<ref>{{cite journal|first=W. C.|last=Fernelius|title=Some problems of inorganic nomenclature|journal=Chem. Eng. News|volume=26|date=1948|pages=161–163|doi=10.1021/cen-v026n003.p161}}</ref><ref>{{cite journal|first1=W. C.|last1=Fernelius|first2=E. M.|last2=Larsen|first3=L. E.|last3=Marchi|first4=C. L.|last4=Rollinson|title=Nomenclature of coördination compounds|journal=Chem. Eng. News|volume=26|issue=8|date=1948|pages=520–523|doi=10.1021/cen-v026n008.p520}}</ref> instead of the original<ref name="1940IUPACinorgnom" /> ''valency''. In 1948 [[Linus Pauling]] proposed that oxidation number could be determined by extrapolating bonds to being completely ionic in the direction of [[electronegativity]].<ref>{{cite journal|first=L.|last=Pauling|title=The modern theory of valency|journal=J. Chem. Soc.|volume=1948|date=1948|pages=1461–1467|doi=10.1039/JR9480001461|pmid=18893624|url=https://authors.library.caltech.edu/59671/|access-date=2021-11-22|archive-date=2021-12-07|archive-url=https://web.archive.org/web/20211207153730/https://authors.library.caltech.edu/59671/|url-status=live}}</ref> A full acceptance of this suggestion was complicated by the fact that the [[Electronegativity#Pauling electronegativity|Pauling electronegativities]] as such depend on the oxidation state and that they may lead to unusual values of oxidation states for some transition metals. In 1990 IUPAC resorted to a postulatory (rule-based) method to determine the oxidation state.<ref>{{cite journal|first=J. G.|last=Calvert|title=IUPAC Recommendation 1990|journal=Pure Appl. Chem.|volume=62|date=1990|page=2204|doi=10.1351/pac199062112167|doi-access=free}}</ref> This was complemented by the synonymous term oxidation number as a descendant of the Stock number introduced in 1940 into the nomenclature. However, the terminology using "[[ligands]]"<ref name="RedBook2005" />{{rp|147}} gave the impression that oxidation number might be something specific to [[coordination complex]]es. This situation and the lack of a real single definition generated numerous debates about the meaning of oxidation state, suggestions about methods to obtain it and definitions of it. To resolve the issue, an IUPAC project (2008-040-1-200) was started in 2008 on the "Comprehensive Definition of Oxidation State", and was concluded by two reports<ref name="10.1515/pac-2013-0505" /><ref name="10.1515/pac-2015-1204" /> and by the revised entries "Oxidation State"<ref name="goldbookoxstate" /> and "Oxidation Number"<ref name="goldbookoxnumber" /> in the [[IUPAC Gold Book]]. The outcomes were a single definition of oxidation state and two algorithms to calculate it in molecular and extended-solid compounds, guided by [[Electronegativity#Allen electronegativity|Allen electronegativities]] that are independent of oxidation state.