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

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

• A. -1
• B. +2
• C. -2
• D. +1

Answer: (C)

Oxygen’s position in the periodic table makes it very electronegative and highly inclined to gain electrons to complete its octet. In oxide ions and the vast majority of oxides, this results in an oxidation number of -2. This bookkeeping matches the rule that the sum of oxidation numbers in a neutral compound is zero (or equals the compound’s charge, for ions), and oxygen commonly carries two extra electrons relative to its neutral state. There are notable exceptions, such as peroxides where each oxygen is -1, or OF2 where oxygen ends up with +2, but those are uncommon special cases. Thus, the standard oxidation number for oxygen in most compounds is -2.

Oxygen’s position in the periodic table makes it very electronegative and highly inclined to gain electrons to complete its octet. In oxide ions and the vast majority of oxides, this results in an oxidation number of -2. This bookkeeping matches the rule that the sum of oxidation numbers in a neutral compound is zero (or equals the compound’s charge, for ions), and oxygen commonly carries two extra electrons relative to its neutral state. There are notable exceptions, such as peroxides where each oxygen is -1, or OF2 where oxygen ends up with +2, but those are uncommon special cases. Thus, the standard oxidation number for oxygen in most compounds is -2.