What is the oxidation number of oxygen in most compounds?

• A. -2
• B. -1
• C. 0
• D. +2

Answer: (A)

Oxygen is assigned an oxidation number of -2 in most compounds because it is highly electronegative and tends to gain two electrons to complete its octet in bonds. In neutral compounds, the total of oxidation numbers must be zero, so oxygen’s -2 is balanced by elements with positive oxidation states. For example, in water, hydrogen is +1 for each H, giving +2 total; oxygen must be -2 to balance to zero. In metal oxides like Na2O, the two sodiums are each +1, totaling +2, so oxygen is -2. There are exceptions, such as peroxides where oxygen is -1 per oxygen, or OF2 where oxygen is +2 because fluorine is more electronegative, but the typical oxidation state for oxygen in most compounds is -2.