What is the common oxidation number of oxygen in most oxides?

What is the common oxidation number of oxygen in most oxides?

• A. 0
• B. -2
• C. +2
• D. -1

Answer: (B)

Oxygen is highly electronegative, so it tends to gain electrons and exist as O2− in most compounds. In a neutral oxide, the total oxidation numbers must balance to zero, so oxygen being −2 requires the other element to provide +2 per oxide unit. That’s why many oxides have the metal in a +2 state and the oxide as a whole is neutral with O contributing −2. There are exceptions, such as peroxides where oxygen is −1, superoxides with an average of −1/2 per oxygen, and compounds like OF2 where oxygen ends up with a positive oxidation state, but for the vast majority of oxides, the oxidation number of oxygen is −2.

Oxygen is highly electronegative, so it tends to gain electrons and exist as O2− in most compounds. In a neutral oxide, the total oxidation numbers must balance to zero, so oxygen being −2 requires the other element to provide +2 per oxide unit. That’s why many oxides have the metal in a +2 state and the oxide as a whole is neutral with O contributing −2. There are exceptions, such as peroxides where oxygen is −1, superoxides with an average of −1/2 per oxygen, and compounds like OF2 where oxygen ends up with a positive oxidation state, but for the vast majority of oxides, the oxidation number of oxygen is −2.