The oxidation number of aluminum in aluminum oxide (Al₂O₃) is +3.
To understand why, let’s break it down. Aluminum oxide consists of two aluminum (Al) atoms and three oxygen (O) atoms. Oxygen typically has an oxidation number of -2. In Al₂O₃, since there are three oxygen atoms, the total contribution from oxygen to the oxidation state is:
3 atoms × (-2) = -6.
For the compound to be neutral, the total oxidation numbers must add up to zero. Therefore, the two aluminum atoms must have a total oxidation state of +6 to balance the -6 from the oxygen:
2 Al (oxidation number) + (-6) = 0.
This means the oxidation number of each aluminum atom must be:
+6 / 2 Al = +3.
Thus, the correct answer is +3.