It denotes the oxidation state of an element in a compound
Rules to assign Oxidation number:
- Elements in free state have zero oxidation number.Ex: H2, O2, N2, C etc. have 0 oxidation number
- For ions composed of only one atom, the charge on the ion is its oxidation number.
Ex: Cl– → -1 oxidation number
Mg2+ → +2
Na+ → +1 - All alkali metals have +1 oxidation number in their compounds. All alkaline earth metals have +2 oxidation number in the compounds.
- Aluminium has +3 oxidation number in all its compounds.
- Oxidation number of oxygen is -2 in most of its compounds.
Ex: in CO2 and H2O, oxidation number of Oxygen is -2.
In Peroxides: Oxidation number of oxygen is -1
Ex: H2O2, Na2O2
In Superoxides: Oxygen has (-½) oxidation number
Ex: KO2, RbO2
Oxygen bound to fluorine:
In OF2, oxidation number of oxygen is +2
In O2F2, oxidation number of oxygen is +1. - Oxidation number of Flourine is always -1.
- Oxidation number of hydrogen is +1 in all compounds except in metallic Hydrides.
In metallic Hydrides, Hydrogen has -1 oxidation state.
CaH2, NaH. - In a compound, the algebraic sum of the oxidation numbers of all its atoms must be zero.
- In a complex ion, the algebraic sum of all the oxidation numbers of its atoms must be equal to the charge on the ion.
Oxidation and Reduction in terms of oxidation number:
- Oxidation – An increase in Oxidation number
Ex: Fe2+ → Fe3+ + e–
Fe2+ is oxidized.
- Reduction – A decrease in the oxidation number.
Ex: Fe3++ e– → Fe2+
Fe3+ is reduced.
Oxidation number/states of representative elements
Group Common oxidation states
1 +1
2 +2
13 +3, +1
14 +4, +3, +2, +1, -1, -2, -3, -4
15 +5, +3, +1, -1, -3
16 +6, +4, +2, -2
17 +7, +5, +3, +1, -1
Fractional oxidation number
It is the average oxidation state of the element in the compound. The element is present in different oxidation states in the compound
For example:
The oxidation number of carbon in propane is – 8/3.
\(H{{-}^{-3}}\underset{H}{\mathop{\underset{|}{\mathop{\overset{H}{\mathop{\overset{|}{\mathop{C}}\,}}\,}}\,}}\,-\underset{H}{\mathop{\underset{|}{\mathop{\overset{H}{\mathop{\overset{|}{\mathop{C}}\,}}\,}}\,}}\,{{\,}^{-2}}-\underset{H}{\mathop{\underset{|}{\mathop{\overset{H}{\mathop{\overset{|}{\mathop{C}}\,}}\,}}\,}}\,{{\,}^{-3}}-H\)
The terminal carbons show – 3 oxidation state and the third carbon in the middle has -2 oxidation so the average oxidation state = -3 + (-3) + (-2) / 3 = -8 / 3.