||It has been suggested that this article be merged into Oxidation state. (Discuss) Proposed since May 2013.|
Oxidation number is used in the nomenclature of inorganic compounds where it is represented by a Roman numeral. In older literature the term is referred to as Stock number, however the use of this term is no longer recommended. Oxidation number represents the degree of oxidation of an element in a substance. This degree of oxidation (often termed the element's oxidation state) is, with the exception of coordination compounds where a different definition applies, the imaginary charge on the atom if the substance were broken down into ions. The oxidation number in compound naming is placed either as a right superscript to the element symbol in chemical formula, for example FeIII, or in parentheses after the name of the element, iron(III) in chemical names. For example Fe2(SO4)3 would be named iron(III) sulfate and its formula could be shown as FeIII2(SO4)3. This is because a sulfate ion has a charge of -2, so each iron atom takes a charge of +3. Note that fractional oxidation numbers should not be used in naming.
In coordination chemistry, the oxidation number of a coordination compound is the charge that the central atom would possess if all the ligands were removed along with the electron pairs that were shared with the central atom.
Determining oxidation numbers 
IUPAC rules 
The 1990 IUPAC rules for determining oxidation state and hence the oxidation number employed in naming have been adopted widely, albeit with some variations, particularly in North America and are as follows:
- The oxidation state of a free element (uncombined element) is zero.
- For a simple (monatomic) ion the oxidation state is equal to the net charge on the ion
- Hydrogen has an oxidation state of +1 and oxygen has an oxidation state of -2 when they are present in most compounds. (Exceptions to this are that hydrogen has an oxidation state of -1 in hydrides of active metals, e.g. LiH, and oxygen has an oxidation state of -1 in peroxides, e.g. H2O2
- The algebraic sum of oxidation states of all atoms in a neutral molecule must be zero, while in ions the algebraic sum of the oxidation states of the constituent atoms must be equal to the charge on the ion.
For example, the oxidation states of sulfur in H2S, S8 (elementary sulfur), SO2, SO3, and H2SO4 are, respectively: –2, 0, +4, +6 and +6.
Sometimes additional rules are added. For example:
Fluorine is always assigned an oxidation number of –1 and this is applied before the rule that oxygen is –2, thus ensuring that for example the oxidation number of oxygen in OF2, oxygen difluoride is correctly calculated as +2.
Using electronegativity 
In the 1970 rules IUPAC recommended that oxidation number was used in nomenclature and elsewhere in inorganic chemistry as the "charge that would be present on an atom if the electrons were assigned to the more electronegative atom", but with a convention that hydrogen is considered to be positive in combination with non-metals and a bond between like atoms makes no contribution to the oxidation number. The oxidation number was represented by a roman numeral.
The idea of using electronegativity in this way was introduced by Pauling in 1947. This method of determining oxidation number is found in some recent text books (example see reference ). This method allows the oxidation number of all atoms in a molecule to be determined whereas the IUPAC 1990/2005 definition does not.
Oxidation number in coordination compounds 
The oxidation number of a central atom in a coordination compound is the charge that it would have if all the ligands were removed along with the electron pairs that were shared with the central atom. The implication is that the oxidation number refers only to the central atom and the oxidation numbers of the ligands cannot be determined.
Note that the 1990 IUPAC definition in the current IUPAC Gold Book seems to imply that oxidation number should only be applied to coordination compounds and was somehow intrinsically different from the oxidation state. This is not the case as later IUPAC recommendations use oxidation number to designate the oxidation state in nomenclature.
Spectroscopic oxidation states vs. formal oxidation numbers 
Although formal oxidation numbers can be helpful for classifying compounds, they are unmeasurable and their physical meaning can be ambiguous. Formal oxidation numbers require particular caution for molecules where the bonding is covalent, since the formal oxidation numbers require the heterolytic removal of ligands, which essentially denies covalency. Spectroscopic oxidation states, as defined by Jorgenson and reiterated by Wieghardt, are measurables that are bench-marked using spectroscopic and crystallographic data.
Oxidation state can also have effect on spectroscopic studies of compounds. In infrared spectroscopy of metal carbonyls this effect is illustrated by using spectroscopic studies on metals from oxidation states of –2 to +2.
History of the oxidation number concept 
The Stock system (named for Alfred Stock who suggested it), was intended to replace the naming that was prevalent at the time. Under the Stock system FeCl2 was called iron(II) chloride rather than ferrous chloride.
The concept of oxidation state is often credited to Latimer in 1938. In 1940 IUPAC recommended that the term Stock number should be replaced by the term oxidation number. In 1947 Pauling proposed that the oxidation number could be determined using the electronegativity of the atoms to determine the "ions" in the formal determination of oxidation number. In 1970 IUPAC defined oxidation number in terms of electronegativity. In 1990 IUPAC changed course and adopted a rule based determination for the "central atom" rather than using electronegativity. This is the definition in the current gold book for "oxidation state". They also introduced the definition of oxidation number, shown in the current gold book, that appears to make oxidation number specific to coordination chemistry. This may not have been their intention as in 2005 they issued new recommendations for inorganic nomenclature that define oxidation number in the same terms as the 1990 definition of oxidation state, and that, oxidation number is, as in the earlier recommendations, used in the naming of inorganic compounds.
Oxidation number versus oxidation state 
In the wider field of chemistry these definitions have not generally been adhered to and both terms are used interchangeably, as they were when Latimer introduced the concept in 1938. For example two well known text books use the term oxidation state and represent it in Roman numerals in chemical formulae. The point has been made that if there is any semantic difference between the terms, then oxidation number refers to the specific numerical value assigned to the entity known as oxidation state, much as IUPAC use the term charge number to refer to the numerical value assigned to the entity know as ionic charge. The IUPAC Gold Book takes the definitions from 1990 IUPAC papers rather than the more recent current IUPAC 2005 recommendations. There is a current IUPAC project, "Towards a comprehensive definition of oxidation state", (project 2008-040-1-200) started in 2009 which has yet to report (March 2013). The project was undertaken because the current definition in the IUPAC Gold Book was seen to be "narrow and circular", and "inapplicable to clusters, Zintl phases and some organometallic complexes".
See also 
- Nomenclature of Inorganic Chemistry IUPAC Recommendations 2005 ed. N. G. Connelly et al. RSC Publishing http://www.chem.qmul.ac.uk/iupac/bioinorg/
- IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "oxidation number".
- IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "oxidation state".
- Loock, Hans-Peter (2011). "Expanded Definition of the Oxidation State". Journal of Chemical Education 88 (3): 282–283. doi:10.1021/ed1005213. ISSN 0021-9584.
- Chemistry, Daniel L. Reger Edward Mercer David W. Ball Scott R. Goode , Cengage Learning (2009), ISBN 0534420125, ISBN 978-0534420123
- Nomenclature of Inorganic chemistry, 2d Edition, Definitive rules 1970, Butterworths
- General Chemistry: An Introduction to Descriptive Chemistry and Modern Chemical Theory, Linus Pauling, W.H Freeman, 1947
- Basic Concepts of Chemistry, 8th Edition, Leo J. Malone, Theodore Dolter, John Wiley & Sons, 2008, ISBN 047174154X , ISBN 978-0471741541
- Bill, E.; Bothe, E.; Chaudhuri, P.; Chlopek, K.; Herebian, D.; Kokatam, S.; Ray, K.; Weyhermueller, T.; Neese, F.; Wieghardt, K. (2005). "Molecular and electronic structure of four- and five-coordinate cobalt complexes containing two o-phenylenediamine- or two o-aminophenol-type ligands at various oxidation levels functional, and correlated ab initio study". Chemistry - A European Journal 11 (1): 204–224. doi:10.1002/chem.200400850. PMID 15549762.
- Jensen, William B. (2007). "The Origin of the Oxidation-State Concept". Journal of Chemical Education 84 (9): 1418. doi:10.1021/ed084p1418. ISSN 0021-9584.
- Cotton, F. Albert; Wilkinson, Geoffrey; Murillo, Carlos A.; Bochmann, Manfred (1999), Advanced Inorganic Chemistry (6th ed.), New York: Wiley-Interscience, ISBN 0-471-19957-5
- Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth–Heinemann. ISBN 0080379419.
- Jensen, William B. (2011). "Oxidation States versus Oxidation Numbers". Journal of Chemical Education 88 (12): 1599–1600. doi:10.1021/ed2001347. ISSN 0021-9584.
- Red Book: IUPAC Nomenclature of Inorganic Chemistry. Third Edition, Blackwell Scientific Publications, Oxford, 1990.
- Calvert, J. G. (1990). "Glossary of atmospheric chemistry terms (Recommendations 1990)". Pure and Applied Chemistry 62 (11): 2167–2219. doi:10.1351/pac199062112167. ISSN 0033-4545.