In magnesium oxide MgO, what are the oxidation numbers of magnesium and oxygen?

• A. Mg +1, O -1
• B. Mg +2, O -2
• C. Mg +3, O -3
• D. Mg 0, O 0

Answer: (B)

In compounds, oxidation numbers reflect how charges balance out in the formula, and for a neutral compound the sum must be zero. In a simple oxide, oxygen almost always has an oxidation state of -2. So for MgO, the oxidation numbers must add to zero: Mg + x plus O at -2 equals 0, which gives x = +2 for magnesium.

Therefore, magnesium is +2 and oxygen is -2. This matches the common behavior of a Group 2 metal forming a +2 cation and oxygen adopting -2 in oxides. The other options would require unlikely or inconsistent charges for magnesium or oxygen in this oxide.