Aluminium oxide is a solid ionic compound, made from atoms of one metal (Aluminum) that have lost three electrons each to become +3 cations, and atoms of a non-metal (oxygen) which have gained two electrons each to become -2 anions.
The numbers 3 and 2 have a lowest common multiple of 6. This means you need TWO aluminum atoms (giving away 2×3=6 electrons total) and THREE oxygen atoms (accepting 3×2=6 electrons total).
Because the electrons are LOST by the metal, and GAINED by the non-metal, there has been a transfer of electrons and this makes it an ionic compound by definition.
You can watch this Lewis Structure get drawn below.
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