Question

Why is the oxidation number of oxygen?

Answer 1

The oxidation number of oxygen can vary.

According to oxidation state rules, any element that is not combined with other elements in a compound is zero.

Neutral compounds have net zero charge, so the charges of elements in a compound must equal zero.

Due to its high electronegativity, oxygen usually has a negative two charge. For example in the compound, calcium oxide, CaO, calcium has a oxidation number of +2 and the oxygen has -2 charge.

In peroxides, such as hydrogen peroxide, `H_2O_2`, each hydrogen has +1 charge, to give a combined oxidation number of +2.
That means that oxygen component, `O_2`,has a combined charge of -2. Consequently, each oxygen must carry a negatives one charge.

Fluorine is the most electronegativity element and when it combines with oxygen to form oxygen difluoride, `OF_2`, the oxidation for each fluorine is negative and the combined oxidation number for two fluorine is negative two. Therefore to balance the charges, oxygen in this case is positive two.