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Construct an Explanation Sapphires and rubies are forms of the mineral
corundum, Al,O3. 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 (A13+) and oxide ions (O2-) is 191 pm. Use Coulomb's law to explain how
the bonding in Al2O3 influences its properties.