The oxidation number (o.n) is an integer ( can be positive or negative )represented in roman numbers (for e.g :IV ) and the role of this number is to help us in balancing reactions and identifying it as an oxidation/reduction reaction.
In conventi0n , we have rules for identifying the oxidation number for elements which are :
1) The oxidation number of an element is equal to zero (O.N = 0 ) .
Examples : – Fe O.n=0
- N2 O.n=0
- O2 O.n= 0
2) The oxidation number (o.n) of a mono-atomic ion is equal to the charge of the ion .
Examples : – H+ , O.n=+I
- N3− , O.n = -III
3) The oxidation number of grouped oxygen is O.n= -II except in peroxides (H2O2 and N2O2 ) the oxidation number of oxygen becomes O.n= -I
Example : – in NaO , O.n(O)= -II
- in H2O2 , O.n(O) = -I
4) The oxidation number of grouped Hydrogen is O.n= +I except for hydrides ( NaH and LiH ) The oxidation number of Hydrogen is O.n=-I
- in HCl , O.n(H) = +I
- in LiH , O.n(H) = -I
5) The oxidation number of grouped elements from group 1 in the periodic table is O.n =+I and that of group two is O.n = +II
- in NaCl , O.n(Na) = +I
- in CaO , O.n(Ca) = +II
6) In a molecule the sum of oxidation number of atoms constituting the molecule is equal to zero
Example : NO2 , O.n(N) + 2[O.n(O)] = 0
O.n(N) -2 = 0
O.n(N) = +II
7) In a polyatomic ion the sum of oxidation number of the ions is equal to the whole charge .
Example : NH4+
O.n(N) + 4[O.n(H)] = +I
O.n(N)+4(+1) = +I
8) The oxidation number of : – Grouped Al = +III
– Grouped F = -I
This was a brief summary about oxidation number and the rules , if you have any questions do not hesitate in commenting or contacting me !