Ionic Bonding (5) Ions and Polarisation

So here is a question for you if you have been following this blog recently.

When the experimentally determined value for the lattice energy is compared with the theoretically calculated values of lattice energy, for some ionic compounds there is a significant difference between the two values.

Why the significant difference?

Let’s first have a look at some data and see if there are any patterns to observe.

Here are the lithium halides

Name	Formula	Theoretical Lattice energy (kJ.mol—1)	Experimental Lattice energy (kJ.mol—1)	Difference (kJ.mol—1)
Lithium Fluoride	LiF	1031	1031	0
Lithium Chloride	LiCl	845	848	3
Lithium Bromide	LiBr	799	803	4
Lithium Iodide	LiI	738	759	21

Here are the silver halides

Name	Formula	Theoretical Lattice energy (kJ.mol—1)	Experimental Lattice energy (kJ.mol—1)	Difference (kJ.mol—1)
Silver Fluoride	AgF	920	958	38
Silver Chloride	AgCl	833	905	72
Silver Bromide	AgBr	816	891	75
Silver Iodide	AgI	778	889	111

The first thing we see is that as the anion increases in size the lattice energy decreases. 

Assuming that these compounds all have the same structure then the influence of the positive ion on the negative ion is decreasing with anion size.

Another thing to note is that these binary compounds all have lattice energies in a similar range around 800 to 900 (kJ.mol^—1)

Thirdly and most importantly, the difference between theoretical and experimental lattice energies increases as the anion increases in size. 

Why is this?

The way the theoretical lattice energy is calculated begins to become less valid as the anion size increases.

Clearly, in the case of the iodides what we assume is being measured is not quite what is being measured at all.

Theoretical values of lattice energy rely on the assumption that ions are either point charges or spherical in shape. 

This spherical model for the shape of the ions is not valid.

Furthermore we find that the properties of the iodides does not fit the ionic model.

For example, the iodides have a significant degree of solubility in organic solvents which would suggest that there is a degree of covalent bonding in these compounds.

How can that be?

One suggestion is that the small, positively charged ions e.g. lithium or silver  polarize the electron clouds of the significantly larger iodide ion. 

Here is a very crude representation of this effect:

Most other diagrams are not much better.

But the point is significant, a region of covalency is created between the ions.

For the first time we see that the “pure” ionic bond or the “pure” covalent bond is a misnomer. 

We have to ask whether they actually exist.

We are going to have to think in terms of partial covalency or partial ionic character.

The ionic or covalent character of a bond will depend on the two atoms in it.

In my next post I’ll discuss this phenomena from the standpoint of a covalent bond that takes on a degree of ionic character.