Understanding the Formula:

* Al₂O₃ represents the chemical formula for aluminum oxide, a compound.

* The subscript "2" next to Al means there are two aluminum atoms in each *formula unit* of aluminum oxide.

* The subscript "3" next to O means there are three oxygen atoms in each *formula unit* of aluminum oxide.

The Key: The number of molecules refers to the number of individual units of a substance. *Formula units* are used for ionic compounds like aluminum oxide, not molecules.

Therefore, there are 3 *formula units* of aluminum oxide in the expression 3Al₂O₃.

To find the number of atoms:

* Aluminum atoms: 3 formula units * 2 Al atoms/formula unit = 6 aluminum atoms

* Oxygen atoms: 3 formula units * 3 O atoms/formula unit = 9 oxygen atoms

Important Note: Aluminum oxide is an ionic compound, not a molecular compound. It exists as a crystal lattice, not individual molecules.