Ari Ben Canaan
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When asked to describe the structure of Aluminium Oxide the markscheme states the following (see attachment).

Now I understand everything except the last statement.

with strong covalent bonding throughout the lattice
I am aware that Al2O3 has a degree of covalent character to it but I've never understood how in the ionic lattice the covalency would make itself known.

Can anyone describe it for me ?
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Kyri
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I think the use of the word "lattice" there was perhaps a poor choice. What they must mean is "strong covalent bonds throughout the giant structure". By giant structure this means something similar to that formed by diamond or SiO2 where the atoms are all held together by covalent bonds rather than intermolecular forces between discrete molecules. Al2O3 has some of this character in its bonding as well some character of an ionic lattice.
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Ari Ben Canaan
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Could I have a second opinion ?
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Ari Ben Canaan
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Anyone ?


(Original post by Plato's Trousers)
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chembob
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(Original post by Kyri)
I think the use of the word "lattice" there was perhaps a poor choice. What they must mean is "strong covalent bonds throughout the giant structure". By giant structure this means something similar to that formed by diamond or SiO2 where the atoms are all held together by covalent bonds rather than intermolecular forces between discrete molecules. Al2O3 has some of this character in its bonding as well some character of an ionic lattice.
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thegodofgod
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Forgive me if I'm wrong, but I've always thought that it would be ionic - 2 Al3+ ions and 3 O2- ions bonded together ionically - two aluminium atom transferring 6 electrons to three oxygen atoms.
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chembob
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(Original post by thegodofgod)
Forgive me if I'm wrong, but I've always thought that it would be ionic - 2 Al3+ ions and 3 O2- ions bonded together ionically - two aluminium atom transferring 6 electrons to three oxygen atoms.
Yes, within an individual molecule that is correct
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Kyri
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(Original post by thegodofgod)
Forgive me if I'm wrong, but I've always thought that it would be ionic - 2 Al3+ ions and 3 O2- ions bonded together ionically - two aluminium atom transferring 6 electrons to three oxygen atoms.
Various texts quote different numbers for this but I've considered electronegativity differences above 2.0 to give mostly ionic character. For example MgO has an electronegativity difference of 2.13 and is an ionic compound. For Al2O3 it is 1.83 so is nearly ionic but not completely. It is certainly significantly polar though.
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ZacNye
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intramolecular bonding and intermolecular bonding are different, so within the molecule there is ionic bonding but between the molecules i.e. within the lattice, there are strong covalent forces holding them together. This is Instead of van-der-waal forces.
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