Assigning Oxidation Numbers
Start with the first rule and work down as necessary!
- The sum of the oxidation numbers of each atom in a molecule must
equal zero; the sum of the oxidation numbers of each atom in an ion
must equal the charge on that ion.
- Elements in their elemental form (atoms for metals, diatomic gases
for H, N, O, etc.) have an oxidation number of 0.
- Single atom ions have an oxidation number equal to their charge.
- Alkali metals have an oxidation number of +1.
- Alkaline earth metals have an oxidation number of +2.
- Hydrogen's oxidation number is +1, unless it is
combined with a metal in which case the oxidation number is -1.
- Fluorine has an oxidation number of -1.
- Oxygen usually has an oxidation number of -2.
Exceptions are -1 in peroxides (O22-), -1/2 in
superoxides (O2-) and -1/3 in ozonides
(O3-)