2.2.2 (b) Ionic Lattice Structure
Syllabus
(b) explanation of the solid structures of giant ionic lattices, resulting from oppositely charged ions strongly attracted in all directions e.g. NaCl
What does this mean?
Again, you don't need to know much more about Ionic Lattices than you did at GCSE.
Ions typically pack into extremely regular crystalline structures, in an arrangement that maximises attractions and minimises repulsions).
For ions of a similar size where the charges match (eg NaCl where we have Na+ and Cl-) the arrangement is the one we have seen before.
The Anions attract all the nearby Cations (and vice versa).
An expanded diagram (left) is more informative.
Because the ions fit together in this manner we can see the Sodium ion (Na+) in the centre (blue) is particularly close to 6 Chloride ions (Cl-) above, below, in front, behind and to the left and right of it.
These are strong attractions due to the closeness.
It is also repelled by the other 12 Sodium ions diagonally across from it in each cube.
But these repulsions are cancelled out by equal attractions to 12 diagonal Chloride ions.
So we can ignore diagonal forces and the lines on the diagram above show the overall attractions only.
We could equally draw the lattice with the Chloride at the centre surrounded by 6 close-neighbouring Sodium ions.
So the NaCl is said to be 6:6 coordinate.
This is really the only lattice that you can be asked to reproduce in the exam but you should probably be aware that it is not a suitable arrangement for all Ionic substances.
Calcium Chloride is CaCl2 - it needs more Chloride ions than Calcium ions.
Though the 2:1 ratio is not obvious in the diagram below.
Caesium Chloride (CsCl) is 1:1 just as Sodium Chloride but Cs+ ions are too large in comparison to Chloride ions.
So, in both cases, different lattice structures would be needed.
In this case the 1:1 ratio is more obvious.
Again, the space-filling model isn't actually very helpful to look at.
Even if it is a more accurate representation of Caesium Chloride's lattice.
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