The charge appears on any atom while present in a molecule is called oxidation number. E.g.
NaCl ————> Na+ + Cl-
(Oxidation number of Na in NaCl = +1)
(Oxidation number of Cl in NaCl = -1)
RULE’S TO FIND OUT OXIDATION NUMBER:
i. Infree statethe number of all atom must be equal to zero e.g. Sº , Cº , Pº
ii. The summation of Oxidation Number of all atoms present in a molecule is always zero.
iii. In any molecule the atom with greater electronegativity assigned -ve sign while the atom w ith lesser electronegativity assigned +ve sign e.g. H+Cl- b/c the electronegativity of Chlorine is greater than Hydrogen.
iv. The oxidation number of oxygen in most of the compounds is -2.
v. The oxidation number of hydrogen attached with metals is -1.
The oxidation number of hydrogen attached with non-metals is +l.
vi. The oxidation number of halogens (F, Cl, Bri, I etc) is -1.
vii. The oxidation number of elements ofGroupIA(Li, Na. K etc) is +1.
The oxidation number of elements of Group IIA (Be, Mg, Ca etc) is +2I
The oxidation number of elements of Group IIIA (B, Al etc) is +3
viii. In polyatomic ion the summation of oxidation number of all atoms is equal to the charge on that molecules for e.g. SO4-2.