Final answer:
The oxidation number of oxygen is typically -2, although in peroxides and compounds with fluorine, oxygen can have an oxidation number of -1 or positive respectively.
Step-by-step explanation:
The oxidation number of oxygen is almost always -2, but there are some exceptions where it differs. Peroxides, such as hydrogen peroxide (H₂O₂), oxygen has an oxidation number of -1. In peroxide ions (O₂⁻), it is also -1. In compounds with fluorine, oxygen can have a positive oxidation number because fluorine is more electronegative and will always have an oxidation number of -1.
For example, in water (H₂O), the hydrogen atoms each have an oxidation number of +1, while the oxygen atom has an oxidation number of -2. This is because oxygen usually attracts electrons more than hydrogen. However, in hydrogen peroxide (H₂O₂), each hydrogen atom still has an oxidation number of +1, but each oxygen atom now has an oxidation number of -1, as the two oxygen atoms share the electrons equally between them, resulting in this exception to the typical oxidation state of oxygen.