This is a useful accounting system for redox with simple rules:
1. All elements have an oxidation state of zero.
2. Hydrogen in compounds is usually 1 ( or +1).
3. Oxygen is usually -2 or -11.
4. Group 1 and 2 elements in compounds are 1 and 2 respectively.
5. Group 6 and 7 elements in compounds are usually -2 or -1 respectively.
6. An element bonded to itself is still 0.
7. The oxidation numbers of the elements in a compound or ion must add up to zero or the charge of the ion.
IMPORTANT
The oxidation number does not imply a charge, e.g. MgO4- the oxidation numbers are Mn(7) and four (4 x-2) giving an overall charge of minus 1. The Mn is NOT 7+.
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