What is the oxidation state of oxygen in most compounds (excluding peroxides or OF2)?

• A. +2
• B. -1
• C. 0
• D. -2

Answer: (D)

Oxygen almost always has an oxidation state of -2 in compounds because it tends to form oxide ions and fill its valence shell. In neutral compounds, the sum of oxidation states is zero, so with typical partners, oxygen takes -2 (as in water or metal oxides). The main exception is when oxygen is bonded to a more electronegative element like fluorine, as in OF2, where oxygen ends up with a positive oxidation state (+2). Peroxides are another special case, where the O–O bond assigns an average oxidation state of -1 to each oxygen. So, for most compounds, the oxidation state of oxygen is -2.