Editing Oxidation state (section)

== IUPAC definition ==

[[International Union of Pure and Applied Chemistry]] (IUPAC) has published a "Comprehensive definition of oxidation state (IUPAC Recommendations 2016)".<ref name="10.1515/pac-2015-1204">{{cite journal|first1=P.|last1=Karen|first2=P.|last2=McArdle|first3=J.|last3=Takats|title=Comprehensive definition of oxidation state (IUPAC Recommendations 2016)|journal=Pure Appl. Chem.|volume=88|issue=8|date=2016|pages=831–839|doi=10.1515/pac-2015-1204|hdl=10852/59520|s2cid=99403810|hdl-access=free}}</ref> It is a distillation of an IUPAC technical report: "Toward a comprehensive definition of oxidation state".<ref name="10.1515/pac-2013-0505">{{cite journal|first1=P.|last1=Karen|first2=P.|last2=McArdle|first3=J.|last3=Takats|title=Toward a comprehensive definition of oxidation state (IUPAC Technical Report)|journal=Pure Appl. Chem.|volume=86|issue=6 |date=2014 |pages=1017–1081|doi=10.1515/pac-2013-0505|doi-access=free}}</ref> According to the IUPAC ''[[Gold Book]]'': "The oxidation state of an atom is the charge of this atom after ionic approximation of its heteronuclear bonds."<ref name=goldbookoxstate>{{GoldBookRef|title=Oxidation state|file=O04365}}</ref> The term ''oxidation number'' is nearly synonymous.<ref name=goldbookoxnumber>{{GoldBookRef|title=Oxidation number|file=O04363}}</ref>

The ionic approximation means extrapolating bonds to ionic. Several criteria<ref name="10.1002/anie.201407561">{{Cite journal|doi = 10.1002/anie.201407561|title = Oxidation State, A Long-Standing Issue!|year = 2015|last = Karen|first = Pavel|journal = Angewandte Chemie International Edition|volume = 54|issue = 16|pages = 4716–4726|pmid = 25757151|pmc = 4506524}}</ref> were considered for the ionic approximation:

# Extrapolation of the bond's polarity; {{ordered list|list_style_type=lower-alpha
| from the electronegativity difference,
| from the dipole moment, and
| from quantum‐chemical calculations of charges.}}
# Assignment of electrons according to the atom's contribution to the bonding [[Molecular Orbital|Molecular orbital]] (MO)<ref name="10.1002/anie.201407561" /><ref>{{cite journal | last=Hooydonk | first=G. Van | title=O n an Ionic Approximation to Chemical Bonding | journal=Zeitschrift für Naturforschung A | volume=29 | issue=5 | date=1974-05-01 | issn=1865-7109 | doi=10.1515/zna-1974-0517 | pages=763–767| bibcode=1974ZNatA..29..763H | doi-access=free }}</ref> or the electron's allegiance in a [[Linear combination of atomic orbitals|LCAO–MO]] model.<ref>{{Cite book|doi=10.1351/goldbook.O04365|chapter=Oxidation state| title=The IUPAC Compendium of Chemical Terminology: The Gold Book|year=2009|isbn=978-0-9678550-9-7}}</ref>

In a bond between two different elements, the bond's electrons are assigned to its main atomic contributor typically of higher electronegativity; in a bond between two atoms of the same element, the electrons are divided equally. Most electronegativity scales depend on the atom's bonding state, which makes the assignment of the oxidation state a somewhat circular argument. For example, some scales may turn out unusual oxidation states, such as −6 for [[platinum]] in {{Chem2|PtH4(2-)}}, for [[Electronegativity#Pauling electronegativity|Pauling]] and [[Electronegativity#Mulliken electronegativity|Mulliken]] scales.<ref name="10.1515/pac-2013-0505" /> The dipole moments would sometimes also turn out abnormal oxidation numbers, such as in [[Carbon monoxide|CO]] and [[Nitrogen oxide|NO]], which
are oriented with their positive end towards oxygen. Therefore, this leaves the atom's contribution to the
bonding MO, the atomic-orbital energy, and from quantum-chemical calculations of charges, as the only viable criteria with cogent values for ionic approximation. However, for a simple estimate for the ionic approximation, we can use [[Electronegativity#Allen electronegativity|Allen electronegativities]],<ref name="10.1515/pac-2013-0505" /> as only that electronegativity scale is truly independent of the oxidation state, as it relates to the average valence‐electron energy of the free atom:

{{Periodic table (electronegativity by Allen scale)|style=font-size:80%}}