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On one hand, covalent character increases the bond dissociation enthalpy (i.e., more energy is needed to be put in) and on the other it lowers the melting point of $\ce{Al2O3}$ to a value lower then would be expected with perfect ionic bonding.

So, does it decrease or increase bond strength?

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  • $\begingroup$ You do realize that you are asking about 2 different energies; the molecular bonds (bond strength) and intermolecular forces (freezing point). $\endgroup$
    – LDC3
    Commented Sep 24, 2017 at 14:27

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In complex terms we can't ever really describe something as been fully ionic or fully covalent but more of a mix of the two. For example, a HCl molecule will have a small amount of ionic bonding character even though we define it as covalent bonding. This is besides the point though... in this case, we generally assume ionic bonds are stronger than covalent.

If you increased the covalent bonding character in a species you are likely to decrease the ionic bonding character resulting in a lower melting point as forces of attraction are now weaker. However, we do see additional bonding occur in species such as vulcanised rubber where you get cross-linking between carbon atoms on adjacent chains. This then obviously increases the melting point as you have not reduced any ionic bonding character

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  • $\begingroup$ Vulcanization of rubber actually increases the size of the molecules, so the melting point is expected to be higher. $\endgroup$
    – LDC3
    Commented Sep 24, 2017 at 14:23
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There was a previous post concerning strength differences between ionic bonds here.

To summarize:

bond strength decrease in the following order covalent > ionic > metallic

Substances with covalent bonds tend to appear weaker than ionically bonded substances (i.e. exhibiting lower boiling points) because ionically bonded substances often form lattice structures from their constituent ions, creating a much more resilient substance, if you will.

You can see the difference in bond strengths, however, when covalent substances form lattice-like structures as well. These network covalent structures are far "stronger" than normal covalent structures because, like ionic substances, they have formed molecular networks that make them stronger. Some examples of these network covalent structures are carbon in the form of diamond or graphene and silicon dioxide (quartz). You will notice that quartz and diamond both exhibit a crystalline structure similar to an ionic substance like NaCl, which is due to their network covalent structures.

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