Oxidation Number or Oxidation State
The oxidation number is defined as a positive or negative number that
represents a charge that an atom appears to have in a given species when the
bonding electrons are counted as per the prescribed set of rules.
Rule 1 : The oxidation number of an atom in an element in its free
uncombined state is zero.
Rule 2 : The oxidation number of a monoatomic ion is same as the
charge on the ion.
The oxidation number for the Calcium ion is +2 and
that of Sulphur in S- 2 is -2
Rule 3 : Oxidation numbers conventionally assigned to atoms in their
chemical compounds are as follows:
a) Oxygen = -2 ( except in peroxides -1)
b) Hydrogen = +1 ( except in metallic hydrides -1 )
c) Group IA elements ( alkali metals ) = +1
d) Group IIA elements ( alkaline earth metals ) = +2
e) Halides = -1
Rule 4 : The algebraic sum of positive and negative oxidation numbers
in compound is zero.
Rule 5 : The algebraic sum of positive and negative oxidation numbers
of atoms in a polyatomic ion is equal to the charge on the ion.
Examples : What is the oxidation number of
1) Mn in Mn2O7
2) S in H2SO4
3) S in S8
4) Br in BrO3-
Considering �x� as oxidation number in each case.
1) 2 (x) + 7 (-2) = 0
\ 2x = 14 and x = 7
2) 2 ( +1 ) + x + 4 (-2 ) = 0
+ 2 + x - 8 = 0 \ x = 6
3) x = 0 ( rule 1 )
4) x + 3 (-2 ) = -1
\ x = -1 + 6 = + 5
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