Polarising effect and lattice enthalpy

I’m confused about this concept and hoping someone can explain what I’m doing wrong

So when we have a small positive ion and a small negative ion they form a strong ionic bond as the outer electron is closer to the nuclei and all that so this increases the lattice enthalpy as we need more energy to break this bond

However, when we have a small positive ion and a LARGE negative ion, there is a larger polarising effect and this polarising effect means that the compound has some covalent character and thus the lattice is stronger and more energy is needed to break the bond

I feel like it’s a contradiction because how are both small and big negative ions resulting in stronger lattice?

I hope the question makes sense

Reply 1

charco

Study Forum Helper

Original post by Aeshakhan

The whole concept of bonding and structure requires a certain degree of mental flexibility.

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Pure ionic structures are giant and there are many strong electrostatic attractions in the lattice.

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Giant covalent structures have many strong covalent bonds holding the lattice together.

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Simple covalent structures have weak forces between individual molecules.

OK?

So if a giant ionic lattice has a tendency towards covalent bonding, i.e. there is polarisation of the electron clouds, then this can strengthen the lattice.
However, if the polarisation leads to the formation of individual molecules then this can weaken the lattice.

You see how the confusion can arise?

It is important to look at the empirical evidence to see what is happening. in the case of AgCl, for example, the lattice appears to be stronger than that predicted by a purely ionic model, whereas with SnCl₂ the lattice appears to be weaker than that predicted by a purely ionic model.

The empirical evidence comes in the form of melting point data.