The rules for determining oxidation numbers are:
- Each atom in a pure element has an oxidation number of zero.
- For monatomic (metal cations) ions, the
oxidation number is equal to the charge on the ion.
- Fluorine always has an oxidation number of -1
in compounds with all other elements.
- Cl, Br, and I always have an oxidation number of
-1 in compounds, except when combined with oxygen or fluorine.
- The oxidation number of H is +1 except in
compounds with metals (hydrides) when H is -1
- The oxidation number of O is -2 except in
peroxides when O is -1
- The algebraic sum of the oxidation number for
the atoms in a neutral compound must be zero. In a polyatomic ion, the sum must
be equal the ion charge.
Remember:
- Oxidation numbers can be fractions
- Oxidation numbers and charges are not the same thing, nor are they written the same
- +2 is oxidation
- 2+ is charge
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