Oxidation Numbers
All elements in their natural state have the oxidation number zero.
ie Hydrogen, H2 oxidation number is 0
Oxidation numbers of the atoms of any molecule add up to zero.
ie Water H2O, sum of oxidation numbers = 0
Oxidation numbers of the components of any ion add up to the charge of that ion.
ie Sulphate, SO4 2- sum of oxidation numbers = -2
The rules in order of priority
1. Oxidation numbers of elements in groups 1, 2 and 3 are always +1, +2, +3
2. The oxidation number of Fluorine is always -1
3. The oxidation number of Hydrogen is usually +1
4. The oxidation number of Oxygen is usually - 2
5. The oxidation number of Chlorine is usually -1
Examples
Deduce the oxidation number of Cl in NaCal
Sum of oxidation numbers must add up to zero.
Therefore Na must be +1
Oxidation number of Cl must balance this
Therefore oxidation state of Cl in NaCl is -1
N2O - O is -2, there are 2 N, so each must be +1
SO4 2- Four O at -2 each gives a total of -8. The ion has an overall charge of -2, so the Oxidation number of S is +6.
CO3 2- Three O at -2 each gives a total of -6. The ion has an overall charge of -2, so the Oxidation number of C is +4.
MnO4 - Four O at -2 each gives a total of -8. The ion has an overall charge of -1, so the Oxidation number of Mn is +7.
Cr2O7 2- Seven O at -2 each gives a total of -14. The ion has an overall charge of -2, so the Oxidation number of each Cr is +6.
NH3 - H is +1, there are 3 H, so N must be -3
KNO3 - O is -2 and K has its normal oxidation number of +1. KNO3 has no overall charge, so N must balance -6 from the 3 O and + 1 from the K. N is +5.