Oxidation state

The oxidation (including oxidation, oxidation value electrochemical value ) indicates the ionic charge of an atom in a chemical compound or a polyatomic anion that would be present if the compound or polyatomic ion would consist of monatomic ions. These bonding electron pairs are mentally assigned to the more electronegative bonding partner. Bonding electron pairs between the same atoms are shared. The atoms in modifications of the elements have an oxidation number of zero. For monatomic ions, the oxidation number is equal to the actual charge. An oxidation number is a numeric value with a sign ( / -).

The oxidation number is a formal size and often has little to do with a real charge. However, it is an important formalism for the stoichiometry of redox reactions.

According to IUPAC, the terms oxidation state and oxidation number can be used. The term oxidation state corresponds to the oxidation described here. The oxidation number is an element of the nomenclature of inorganic salts (for example, iron ( III) chloride ), and complex compounds (eg, for Kaliumhexacyanidoferrat (II )), and complex ions. In the nomenclature of compounds ( oxidation number) only integer oxidation numbers are used and displayed next to the number 0 only in Roman numerals. In complex compounds of the value indicates the oxidation number of the central atom.

Another definition is: The oxidation number of an atom in a chemical compound is formally a measure for indicating the conditions of the electron density at that atom. A positive oxidation number indicates that the electron density is reduced in comparison with its normal state, a negative indicates that the electron density is increased by the atom.


Oxidation numbers are used in redox reactions to better identify the processes. A decrease in the oxidation number of an element by means of a redox reaction, that this element has been reduced, an increase in the analog means oxidation number of an element, that it has been oxidized.

Indication of the oxidation number

The oxidation numbers are usually indicated to display formulas as Arabic numerals and are integers in the rule. In contrast to ionic charges, the plus and minus signs are specified before the numbers. For the preparation of redox reactions, oxidation numbers are above an element symbol. Occasionally, however, Roman numerals are used. The possible oxidation numbers of the chemical elements are listed here.

For formulating redox reactions only the oxidation numbers of the crucial elements in the reaction are often given:

The oxidation numbers can also assume fractional values ​​. Hyper oxides, such as potassium superoxide ( KO2 ), the oxygen atoms have an oxidation number of 0.5, and differ from the peroxides in the oxidation number -1.

In Fe3O4 (iron (II, III ) oxide ) has an average iron oxidation number of 8 / 3 The oxidation shown in Roman numerals in the name indicate that iron atoms are present in this compound with the oxidation states 2 and 3. ( It is not the oxidation state 2.3 ago. ) FeIIFe2IIIO4 has an inverse spinel structure ( Simplified: FeO · Fe2O3) and formal Fe2 - and Fe3 ions can be localized.

The thiosulfate ion ( S2O32 -, disulfate ( II) ion ) consists of two dissimilar sulfur atoms. The average oxidation state of the sulfur is 2. The discrete steps, which can be derived from the structure, 5 and -1. For stoichiometric calculations of inorganic chemistry are, the middle and the discrete oxidation states.

In organic compounds, the oxidation numbers for each carbon atom are determined separately:

By comparing the oxidation numbers can be seen, for example, that a conversion of a primary alcohol to an aldehyde or the reaction of an aldehyde to a carboxylic acid are oxidations.

In organic reactions, a simple stoichiometric considerations do not meet most to describe reaction. Therefore, oxidation states play a minor role compared to inorganic chemistry. A stoichiometric reaction proceeding, for example, the Tollensprobe:

In principle the sum of the oxidation numbers of the atoms of a molecular compound is zero. For ions, the sum of the oxidation numbers is equal to the ionic charge. At stoichiometric redox properly prepared, the sum of the oxidation numbers of the reactants is equal to the sum of the oxidation numbers of the products.

Determination of oxidation number

Main rules

The oxidation number can be derived using the following rules:

Auxiliary rules

In practice, it has proved useful to formulate some rules for the determination of oxidation numbers:

Determination based on the electronegativity

The oxidation numbers can be when a Lewis structure of the molecule is present, easily determined from the electronegativity of the respective elements. It splits to each bond and mentally calculated that nuclear would get the bonding electrons then. This is dependent on the electronegativity; the atom with the greater electronegativity gets the bonding electrons. This changes the charge of the atom, the load corresponds to the oxidation number. The columns of the bonds is only a mind game, the bonds are not actually split.

The chart on the right shows an example of the procedure for determining the oxidation numbers of the atoms of the 5- hydroxycytosine molecule. As an example, here is the procedure on the carbon atom can be explained by the oxidation number ± 0: This carbon atom forms three bonds to adjacent atoms of to nitrogen, hydrogen and a double bond to another carbon atom. Now the electronegativities of these elements are compared; Carbon has an electronegativity of 2.55.

  • Nitrogen has an electronegativity of 3.04. As this is greater than that of the carbon, of the nitrogen would get in the case of an imaginary cleavage of the bond both the bonding electrons.
  • Hydrogen has an electronegativity of 2.2. As it is smaller than that of carbon, the carbon would get in the case of an imaginary cleavage of the bond both the bonding electrons.
  • The upper carbon atom has of course also an electronegativity of 2.55. Therefore, the two carbon atoms in the case of an imaginary parts cleavage of the bond the bonding electrons. There is a double bond, both would get two.

Adds thus gets the carbon atom four bonding electrons. Elemental carbon also has four valence electrons, its charge has thus not changed by the imaginary cleavage. Its oxidation number is 0

In comparison, the nitrogen atom will receive six lowest bonding electrons in the case of an imaginary split (two each of the two carbon atoms and two of the hydrogen atom). Elemental nitrogen has only three valence electrons. Because electrons are negatively charged, therefore, the nitrogen atom possessed by the imaginary splitting the charge -3. It must therefore also its oxidation number.

To check all derived oxidation numbers can be added. Their sum must equal zero result if the overall molecule is uncharged.