How do you find the oxidation numbers for compounds?
By remembering some important rules.
(1) The total charge of a stable compound is always equal to zero (meaning no charge). For example, the
(2) If the substance is an ion (either there is a positive or negative charge) the total oxidation state of the in is the charge (i.e. oxidation state of
(3) All elements from Group 1A has an oxidation state of +1. (e.g.
(4) All Group 2A and 3B elements have an oxidation state of +2 and +3, respectively. (e.g.
(5) Oxygen always have a charge -2 except for peroxide ion (
(6) Hydrogen always have a charge of +1 if it is bonded with a non-metal (as in the case of
If you can remember and apply these rules, assigning an oxidation state for a given element would be easy for you.
Let's try an example. Find the oxidation state of
Following the rules above and converting the atoms according to their oxidation states,
H : (+1)(2 atoms) = 2
This based on Rule 6 wherein the
O : (-2) (4 atoms) = -8
Again, based on the Rule 5. The oxidation state is multiplied by the number of atoms based on the subscript.
Getting the sum, we have
Therefore, the oxidation state of