resist
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I'm currently revising CHEM5 and I seem to have hit some conflicting information in my text book.

According to the book experimental lattice enthalpies are higher than theoretical values when there is partial covalent bonding, i.e when the purely ionic model doesn't fit. It goes on to say that the covalent character creates stronger bonding.

However a few pages later in the chapter on Period 3 Oxides my book says "Al2O3 has a lower melting point than you might expect because the 3+ ions distort the oxygen's electron cloud making the bonds partially covalent".

This is completely conflicting with the pure ionic model is it not? Can anyone help me out here?
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charco
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(Original post by resist)
I'm currently revising CHEM5 and I seem to have hit some conflicting information in my text book.

According to the book experimental lattice enthalpies are higher than theoretical values when there is partial covalent bonding, i.e when the purely ionic model doesn't fit. It goes on to say that the covalent character creates stronger bonding.

However a few pages later in the chapter on Period 3 Oxides my book says "Al2O3 has a lower melting point than you might expect because the 3+ ions distort the oxygen's electron cloud making the bonds partially covalent".

This is completely conflicting with the pure ionic model is it not? Can anyone help me out here?
Yes, it is a confusing issue.

A degree of covalency leads to weaker bonding in general. However there are cases where the ionic model appears to break down (as far as lattice enthalpy goes) and there is stronger bonding.

My rationale is that there are two forms of covalent bonding:

1. one that creates individual molecules
2. one that creates a giant lattice

It is easy to say that the lower m.p. of lithium chloride is due to a degree of covalency, but that the higher experimental lattice enthalpy of AgCl (cf the theoretical) is due to some extra bonding not envisaged by the theory.

Occasionally chemistry is like this. Make the observation and then adjust the theory to fit ...
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resist
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Thanks for your help. I guess I'll have to watch out for any questions on this in the exam and think them through carefully.
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