Project Details Towards a comprehensive definition of oxidation state

Project No.:
Start Date:
01 January 2009
End Date:
12 December 2017
Division Name:
Inorganic Chemistry Division
Division No.:


The objective of this project is to obtain a comprehensive definition of the term oxidation state for an atom in a molecular entity or extended structure, and to suggest an algorithm to evaluate its numerical value in an unambiguous manner. A glossary of terms related to oxidation and reduction will be created.


Based on postulated oxidation states for oxygen and hydrogen and on the term oxidation per se, the definition of the oxidation state in the IUPAC Gold book is narrow and circular. It treats hydrides and O?O compounds as exceptions and is not applicable to clusters, Zintl phases and some organometallic complexes. A further deficiency has been identified by E. Vitz in J. Chem. Ed. 79 (2002) 397; the term oxidation state is based on terms oxidation and reduction, which in turn are defined as an actual removal and addition of electrons. Because the latter is not always the case, a remedy is suggested in making the oxidation state a primary term and relating the definitions of oxidation and reduction to it.

This project will review and evaluate the current definitions of oxidation state in textbooks, teaching pamphlets, and articles with a view to determining how they serve as a primary definition in redox chemistry. These electron-counting schemes will be scrutinized for universality on chemical entities, such as uncharged and charged atoms and molecules (simple or cluster-like, ions or radicals), as well as extended structures, in order to see whether or not a simple algorithm is applicable and actually implied by the definition. Such a defined oxidation state will then be used to create a glossary of related, more specialized terms.

A preprint will be circulated to a list of inorganic chemists before being submitted as Recommendation for Pure and Applied Chemistry. The relevant definitions in the IUPAC Gold Book web site will be updated.


> project announcement published in Chem. Int. May-June 2009

Sep 2011 update: The experimental part of the project is finished (the research, analysis, data gathering, calculations, and discussions of about 100 examples among 3 task group members). The write-up of the Technical Report is almost complete and currently being refined. Write-up of a Technical Repot will be started soon. Write-up of a pedagogical summary for Journal of Chemical Education will be commenced soon.

March 2014 update: The Technical Report is being finalized and will appear in Pure Appl. Chem. (doi:10.1515/pac-2013-0505)

July 2014 update: Final reference: PAC, Vol 86, No 6, pp 1017-1081.

March 2015 update: Pavel Karen published an Essay entitled “Oxidation State, A Long-Standing Issue!” in Angew. Chem. Int. Ed. (doi:10.1002/anie.201407561)

January 2016 update – A manuscript titled ‘Comprehensive Definition of Oxidation State’ is available as provisional recommendations and for public review until 31 May 2016.

October 2016 update – the ‘Comprehensive definition of oxidation state (IUPAC Recommendations 2016)’ is published as PAC 88(8), pp. 831-839, 2016; (online 19 Oct 2016)

The last tasks to be completed are to edit the Oxidation-State entry in the Goldbook and Wikipedia. The Wikipedia entry is based on the current GoldBook definitions which in turn must be updated first. The extent of the Wikipedia entry revision is some 4 pages of text, of which 3 would describe the Gold-Book entries as such, the rest being minor changes in details of the text that follows. At present, the task group is preparing these contributions.

June 2017 update – The Gold-Book entries for oxidation state ( and oxidation number
( have been updated.

December  2017 update – In October we edited the entire Wikipedia entry <> according to the IUPAC Recommendations, and then we spent a few weeks by checking and correcting details in the text, as well as responding to edits and comments by other Wikipedia editors. We also edited the Oxidation State entry in the corresponding Simple Wikipedia <>.

Project completed
Page last updated: 13 Dec 2017