CBSE Class 11-science Answered

Rule 1: The oxidation number of an atom in free uncombined elemental state is zero. For example, each atom in the elements such as Ca, H₂, Cl₂, O₃, S₈, P₄ and so on has oxidation number of zero.

Rule 2: The oxidation number of an atom in a monoatomic ion is equal to its charge.

For example, Ba⁺²   Cr⁺³     K⁺¹

                    ↑          ↑        ↑

                     +2        +3    +1

Rule 3: The oxidation number of H atom is either +1 or -1. When H is bonded to non-metals, its oxidation number is +1. When it is bonded to metals, its oxidation number is -1.

For example:

[O – H ]^-       [Li – H]

↑      ↑              ↑       ↑

-2   +1           +1     -1

Rule 4: The oxidation number of oxygen is usually -2 in all of its compounds except in peroxides and peroxide ions where it has oxidation number of -1.

For example:

Ca –O        H – O – O – H

↑      ↑        ↑     ↑      ↑     ↑

+2 -2        +1  -1    -1  +1

Rule 5 :The oxidation number of F is always -1 in all of its compounds. The other halogens  Cl, Br and I usually have oxidation number of -1 in their compounds. The exception is the compounds in which these halogens (Cl, Br and I) are bonded to oxygen. In such compounds, oxidation number of Cl, Br or I is +1.

For example:

H – F     Cl – O – Cl

↑    ↑      ↑      ↑     ↑

+1  -1   +1  -2   + 1

Rule 6:The algebraic sum of the oxidation states of all atoms in a neutral molecule is zero. For example, IN CdS oxidation number of Cd+ oxidation number of S = +2-2 = 0

Rule 7: The algebraic sum of the oxidation numbers of all the atoms in a polyatomic ion is equal to the net charge of the ion. In SO₄^2- ion,

Oxidation state of S + 4 x Oxidation state of oxygen

=+6 + 4x (-2) = 6 – 8 = -2