What is the oxidation number of oxygen in H2O?
Oxygen almost always has an oxidation number of -2, except in peroxides (H 2 O 2) where it is -1 and in compounds with fluorine (OF 2) where it is +2. Hydrogen has an oxidation number of +1 when combined with non-metals, but it has an oxidation number of -1 when combined with metals.
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