# Why is the oxidation number of oxygen?

Jun 17, 2014

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, ${H}_{2} {O}_{2}$, 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, $O {F}_{2}$, 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.