Difference between the experimental (Born-Haber) and theoretical lattice energies

Can the the experimental (Born-Haber) lattice energy ever be greater than the theoretical lattice energies (in other words can the BH lattice energy be less negative than the theoretical lattice energy?). Or can it only be more negative..

It can be both greater and smaller...

Would they ask questions (Edexcel) on situations where the BH is greater? Or is this unlikely?

Thanks

You are more likely to be asked why you think the two values are different.

The simple answer is that the purely theoretical ionic model assumes perfect ionic (electrostatic) forces (bonding).

The fact that the theoretical values differ from the experimental (Born-Haber) values suggests that the bonding within the lattice is not purely ionic.

I don't think that you need to go beyond that. You don't really need to go into a discussion of the actual bonding, because, in essence, it probably is not known for certain.

You are more likely to be asked why you think the two values are different.

The simple answer is that the purely theoretical ionic model assumes perfect ionic (electrostatic) forces (bonding).

The fact that the theoretical values differ from the experimental (Born-Haber) values suggests that the bonding within the lattice is not purely ionic.

I don't think that you need to go beyond that. You don't really need to go into a discussion of the actual bonding, because, in essence, it probably is not known for certain.

When you say ' experimental (Born-Haber) values ' is that synonymous with lattice enthalpy of formation?

Edit: Also, it says that there is a tendency towards covalent character if the positive or negative ion has multiple charges. What does it mean by 'multiple charges' ?