Oxidation Number Concept

It denotes the oxidation state of an element in a compound

Rules to assign Oxidation number:

1. Elements in free state have zero oxidation number.Ex: H₂, O₂, N₂, C etc. have 0 oxidation number
2. 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
3. All alkali metals have +1 oxidation number in their compounds. All alkaline earth metals have +2 oxidation number in the compounds.
4. Aluminium has +3 oxidation number in all its compounds.
5. 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: H₂O_2, 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.
6. Oxidation number of Flourine is always -1.
7. Oxidation number of hydrogen is +1 in all compounds except in metallic Hydrides.
   In metallic Hydrides, Hydrogen has -1 oxidation state.
   CaH₂, NaH.
8. In a compound, the algebraic sum of the oxidation numbers of all its atoms must be zero.
9. 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.