how to calculate oxidation number - Overview, Structure, Properties & Uses

How to Calculate Oxidation Number - Explanation, Examples, FAQs

Edited By Team Careers360 | Updated on Jun 30, 2022 07:27 PM IST

What is meant by oxidation state (oxidation number)?

Oxidation number of an element can be defined as the charge which an atom of the element has or appears to have when present in the combined state in a compound.

Electrons shared between two like atoms are divided equally between the sharing atoms.
Electrons shared between two unlike atoms are counted with the more electronegative atom.
Atoms can assume positive, zero or negative values of oxidation numbers depending on their state of combination. Oxidation number can be a fraction in some cases.
Oxidation number calculator can be used to calculate the oxidation number online.

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Oxidation number rules.

Oxidation number is generally defined as the charge assigned to the atom in a species according to some arbitrary rules.

. The oxidation number of an element in the free or elementary state is always zero.

For example, oxidation number of oxygen or oxidation state of oxygen in O₂ is zero and that of iron in Fe is zero.

. The oxidation number of an element in monoatomic ion is equal to the charge on the ion.

For example, in KCl, the oxidation number of K⁺ is +1 and that of Cl^- is -1. The oxidation number of Ca²⁺ ion is +2 and that of Al³⁺ is +3.

. The oxidation number of fluorine is always -1 in all its compounds.

. Hydrogen is assigned oxidation number +1 in all its compounds except in metal hydrides. In metal hydrides like NaH, MgH₂, CaH₂, LiH etc. the oxidation number of hydrogen is -1.

. In compounds containing oxygen, the oxidation number of oxygen is -2 except in peroxides such as Na₂O₂ and in OF₂. The oxidation number of oxygen in peroxides Na₂O₂is -1 and in OF₂, the oxidation number is +2.

. The oxidation number of alkali metals is +1 in all their compounds and the oxidation number of alkaline earth metals is +2 in all their compounds.

. The algebraic sum of the oxidation numbers of all the atoms in a neutral molecules is zero. In case of complex ion, the sum of the oxidation numbers of all the atoms is equal to the charge on the ion.

For example, in SO^2-₄(sulphate ion), the sum of the oxidation number of sulphur atom and four oxygen atoms will be equal to -2.

. In binary compounds of metal and non-metal, the oxidation number of metal is always positive while that of the non-metal is negative. For example, in Kl, The oxidation number of potassium is +1 and that of iodine is -1.

. In binary compounds of non-metals, the more electronegative atom has negative oxidation number and the less electronegative atom has positive oxidation number. For example, the oxidation number of chlorine in ClF₃is +3 while that in ICI it is -1.

The oxidation number of carbon in organic compounds can be calculated using the following general method. If the number of hydrogen atoms in the compound is n_H, the number of oxygen atoms is n_º and the number of carbon atoms is n_C, then

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Where ‘x’ is the oxidation number of carbon.

Oxidation state of a metal in a compound is sometimes represented by stock notation. According to this, the oxidation number is written as roman numeral in parenthesis after the symbol of the metal in the molecular formula, For example-

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Calculating the oxidation number of some elements

How to calculate the oxidation number of ‘Cr’ in K₂Cr₂O₇and CrO^2-₄.

Let us take the oxidation number of ‘Cr’ in both compounds as ‘x’.

In K₂Cr₂O₇ the sum of the oxidation numbers of all atoms is given as:

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That is the oxidation number of ‘Cr’ in K₂Cr₂O₇ is +6.

The oxidation number of ‘Cr’ in CrO^2-₄ is given as:

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That is the oxidation number of ‘Cr’ in CrO^2-₄ is +6.

Calculating the oxidation number of sulphur in Na₂S₄O₆ and S₂O^2-₈.

Let us take the oxidation number of sulphur in both compounds as ‘x’. Then the sum of the oxidation numbers of all atoms is given as:

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That is the oxidation number of ‘S’ in Na₂S₄O₆is 2.5.

In S₂O^2-₈ ion, there is a peroxide bond (O-O) and the two oxygen atoms show oxidation number -1 each. The remaining six oxygen atoms show -2 each.

The oxidation number of ‘S’ in S₂O^2-₈is shown as:

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Calculating the sum of coordination and oxidation number of metal atom or how to calculate oxidation number of metal?

The coordination number also called ligancy of the central atom or ion in a complex is the total number of ligand donor atoms directly coordinated to it. In other words, coordination number is the total number of sites on the central metal atom or ion at which the ligands are attached through coordinate bonds.

In the complex [M(en)₂(C₂O₄)]Cl, the molecules that are present in the coordination sphere are two ions of ethylenediamine and 1 ion of oxalate. Here the Cl^- is an anion and it exists outside the coordination sphere. Both of the ligands attached to the metal atom are bidentate ligands.

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Then the coordination number of the complex is:

Here ethylenediamine is a neutral molecule and it has zero oxidation state whereas the oxidation number of oxalate ion is -2. Let us take the oxidation number of the metal atom as ‘x’. Then,

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