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What's the effect of bond polarity on the perfect ionic model?

(edited 5 years ago)

What you have to consider is the explanation of what affects melting point is more involved than what you have here and the structure of the compound will have a large effect as well (structures of compounds like Al2O3 can be devilishly complicated) and when they melt in reality their ions do not just separate but the liquid has a structure.
You also have to realise that when the covalent character becomes significant enough the compound will behave like a simple molecular substance
eg AlCl3 is not Al3+ and Cl- ions but AlCl3 molecules and so has a low melting point because you are separating the molecules not the ions (in this case the liquid phase consists of Al2Cl6 dimers) but MgCl2 is more ionic and has a higher melting point.

(edited 5 years ago)

Original post by sbzk101

Thanks! But am I right in thinking that some (but not enough to cause a molecular structure) covalent character DOES increase the melting point by a certain amount?

Some covalent character will increase the lattice enthalpy. For example in AgCl the lattice enthalpy is around 100 kJmol-1 more than if it was considered pure ionic.
Substances with higher lattice enthalpies may well have higher melting but it isn't that simple as other factors such as structure play a large role.