gives the degree of oxidation of an atom in terms of counting electrons. The higher the oxidation state (OS) of a
given atom, the greater is its degree of oxidation
OS of an atom is the charge of this atom after ionic approximation of its heteronuclear bonds.
The underlying principle is that the ionic sign in an AB molecule is deduced from the electron allegiance
in a LCAO-MO model: The bond’s electrons are assigned to its main atomic contributor. Homonuclear AA
bonds are divided equally. In practical use, the ionic-approximation sign follows Allen electronegativities
(see Source). There are two general algorithms to calculate OS:
Algorithm of assigning bonds, which works on a Lewis formula
showing all valence electrons in a molecule: OS equals the charge of an atom after its heteronuclear
bonds have been assigned to the more electronegative partner (except when that partner is a reversibly
bonded Lewis-acid ligand) and homonuclear bonds have been divided equally:
Algorithm of summing bond orders: Heteronuclear-bond orders are summed at the atom as positive if
that atom is the electropositive partner in a particular bond and as negative if not, and the
atom’s formal charge (if any) is added to that sum, yielding the OS. This algorithm works on
Lewis formulas and on bond graphs of atom connectivities for an extended solid:
- Specific uses may require modified OS values: Electrochemical OS is nominally adjusted to
represent a redox-active molecule or ion in Latimer or Frost diagrams. Nominal OS values
may also be chosen from close alternatives for systematic-chemistry descriptions.
- Some OS may be ambiguous, typically when two or more redox-prone atoms enter bonding
compromises and nearest integer values have to be chosen for the OS.
- The caveat of reversibly bonded Lewis-acid ligands originates from the simplifying
use of electronegativity instead of the MO-based electron allegiance to decide the ionic sign.
Typical examples are the transition-metal complexes with so called Z ligands in the
CBC ligand-classification scheme (see Source).
- When used in chemical nomenclature as a symbol, the OS value is in roman numerals.
- At the introductory teaching level, prior to the bonding-based definition and algorithms:
OS for an element in a chemical formula is calculated from the overall charge and
postulated OS values for all the other atoms. For example, postulating OS = +1 for H and −2
for O yields correct OS in oxides, hydroxides, and acids like H2SO4; with coverage
extended to H2O2 via decreasing priority along the sequence of the two
postulates. Additional postulates may expand the range of compounds to fit a textbook’s scope.
IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by
A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997).
XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic,
J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8.