Editing Oxidation state (section)

=== Fractional oxidation states ===
Fractional oxidation states are often used to represent the average oxidation state of several atoms of the same element in a structure. For example, the formula of [[magnetite]] is {{chem|Fe|3|O|4}}, implying an average oxidation state for iron of +{{sfrac|8|3}}.<ref name=Petrucci>{{cite book|first1=R. H.|last1=Petrucci|first2=W. S.|last2=Harwood|first3=F. G.|last3=Herring|title=General Chemistry|url=https://archive.org/details/generalchemistry00hill|url-access=registration|edition=8th|publisher=Prentice-Hall|date=2002|isbn=978-0-13-033445-9}}{{ISBN missing}}</ref>{{rp|81–82}} However, this average value may not be representative if the atoms are not equivalent. In a {{chem|Fe|3|O|4}} crystal below {{cvt|120|K|°C|0}}, two-thirds of the cations are {{chem|Fe|3+}} and one-third are {{chem|Fe|2+}}, and the formula may be more clearly represented as FeO·{{chem|Fe|2|O|3}}.<ref>{{cite journal|first1=M. S.|last1=Senn|first2=J. P.|last2=Wright|first3=J. P.|last3=Attfield|title=Charge order and three-site distortions in the Verwey structure of magnetite|journal=Nature|volume=481|issue=7380|pages=173–6|date=2012|doi=10.1038/nature10704|pmid=22190035|bibcode=2012Natur.481..173S|s2cid=4425300|url=https://www.pure.ed.ac.uk/ws/files/10796489/Charge_order_and_three_site_distortions_in_the_Verwey_structure_of_magnetite.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://www.pure.ed.ac.uk/ws/files/10796489/Charge_order_and_three_site_distortions_in_the_Verwey_structure_of_magnetite.pdf |archive-date=2022-10-09 |url-status=live|hdl=20.500.11820/1b3bb558-52d5-419f-9944-ab917dc95f5e|hdl-access=free}}</ref>

Likewise, [[propane]], {{chem|C|3|H|8}}, has been described as having a carbon oxidation state of −{{sfrac|8|3}}.<ref>{{cite book|first1=K. W.|last1=Whitten|first2=K. D.|last2=Galley|first3=R. E.|last3=Davis|title=General Chemistry|url=https://archive.org/details/generalchemistry00whit_0|url-access=registration|edition=4th|publisher=Saunders|date=1992|page=[https://archive.org/details/generalchemistry00whit_0/page/147 147]|isbn=978-0-03-075156-1}}{{ISBN missing}}</ref> Again, this is an average value since the structure of the molecule is {{chem|H|3|C−CH|2|−CH|3}}, with the first and third carbon atoms each having an oxidation state of −3 and the central one −2.

An example with true fractional oxidation states for equivalent atoms is potassium [[superoxide]], {{chem|KO|2}}. The diatomic superoxide ion {{chem|O|2|−}} has an overall charge of −1, so each of its two equivalent oxygen atoms is assigned an oxidation state of −{{sfrac|1|2}}. This ion can be described as a [[resonance (chemistry)|resonance]] hybrid of two Lewis structures, where each oxygen has an oxidation state of 0 in one structure and −1 in the other.

For the [[cyclopentadienyl anion]] {{chem|C|5|H|5|−}}, the oxidation state of C is −1 + −{{sfrac|1|5}} = −{{sfrac|6|5}}. The −1 occurs because each carbon is bonded to one hydrogen atom (a less electronegative element), and the −{{sfrac|1|5}} because the total ionic charge of −1 is divided among five equivalent carbons. Again this can be described as a resonance hybrid of five equivalent structures, each having four carbons with oxidation state −1 and one with −2.

:{| class="wikitable"
|+ Examples of fractional oxidation states for carbon
|-
! Oxidation state !! Example species
|-
| −{{sfrac|6|5}} || [[Cyclopentadienyl anion|{{chem|C|5|H|5|−}}]]
|-
| −{{sfrac|6|7}} || [[tropylium|{{chem|C|7|H|7|+}}]]
|-
| +{{sfrac|3|2}} || [[Squarate ion|{{chem|C|4|O|4|2−}}]]
|}

Finally, fractional oxidation numbers '''are not  used''' in the chemical nomenclature.<ref name="RedBook2005">{{cite book|first1=N. G.|last1=Connelly|first2=T.|last2=Damhus|first3=R. M.|last3=Hartshorn|first4=A. T.|last4=Hutton|title=Nomenclature of Inorganic Chemistry (IUPAC Recommendations 2005)|publisher=RSC Publishing|url=http://www.old.iupac.org/publications/books/rbook/Red_Book_2005.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.old.iupac.org/publications/books/rbook/Red_Book_2005.pdf |archive-date=2022-10-09 |url-status=live}}</ref>{{rp|66}} For example the red lead [[Lead(II,IV) oxide|{{chem|Pb|3|O|4}}]] is represented as lead(II,IV) oxide, showing the oxidation states of the two nonequivalent [[lead (metal)|lead]] atoms.