Sapphires and rubies are forms of the mineral
corundum, Al₂O3. Corundum is one of the hardest known natural materials. The
hardness of ionic compounds depends on the structure of the crystal and the
strength of the ionic bonds. The distance between the aluminum ions (Al³+) and
oxide ions (O²-) is 191 pm. Use Coulomb's law to explain how the bonding in Al₂O3
influences its properties.