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Aluminum oxide is an ionic compound, formed by the transfer of electrons from the aluminium to oxygen atoms. This means that the atoms of aluminum are losing three electrons each to become +3 cations while the atoms of oxygen gain two electrons to become -2 anions.
The difference in the electronegativity of a metal compared to the non-metal can be used as a guide for deciding whether the bonding between the two is ionic or covalent. If the difference is greater than 1.5 units then the bonding is ionic.
If the difference is less than 1.5 then the bonding is covalent. This is a rough approximation and can be difficult to find exceptions.
Valence Electrons in Al2O3
The total valence electrons available on the Aluminium and Oxygen atoms in the Al2O3 lewis structure are 24.
There are 12 bond pairs forming 2 single covalent bonds and 2 double bonds between the Al and O atoms. There are also 12 lone pair electrons that are placed on the O atoms as unshared electrons.
Lone pair electrons are a non-bonding electron or an electron that is not involved in any covalent bond between an atom or a molecule. This is the reason why lone pair electrons are called lone pairs.
Lewis Structure for Al2O3
In this structure we can see that the lone pair electrons on the O atoms are displaced from their bonds with the Al atoms to form the lone pair ions. It is also important to note that the lone pair ions are bonded with the valence shell electrons of the O atoms and not the central Al atoms. It is this type of lone pair electron bonding that makes the Al2O3 lewis structure a solid polar ionic compound.