In most compounds, what is the oxidation number of oxygen?

• A. -1
• B. -2
• C. 0
• D. +2

Answer: (B)

Oxidation numbers are a way to assign a notional charge to atoms in compounds to keep track of electron transfer during reactions. Oxygen almost always has an oxidation number of -2 because it is highly electronegative and typically gains two electrons to complete its octet, forming the O2− ion in many compounds. This -2 value reflects its common bonding behavior with other elements, where it pulls electron density toward itself.

There are exceptions worth noting: in peroxides (like H2O2) the presence of an O–O bond means each oxygen effectively has -1; in fluorine-containing compounds such as OF2, oxygen is more oxidized and has +2; and in the elemental form O2, the oxidation number is 0. But in the vast majority of compounds, -2 is the standard oxidation state for oxygen.