Foundation and higher tier
Structure of ionic compounds
The positive metal ions and the negative ions produced when metals and non-metals react will be attracted to each other and pack as closely together as possible in a regular way. Structures with regular arrangements of atoms or ions are often referred to as being crystalline. The electrostatic attraction of positive and negative ions is called an ionic bond. Ionic compounds consist of a giant structure made of these ions. These giant structures of ions are often referred to as an ionic lattice. The smaller the ions and the more highly charged they are will mean that they can pack together more closely and form a stronger ionic bond. Sodium a group I metal will form an ion with a +1 charge. Chlorine a group 7 halogen will form an ion with a -1 charge. If you look at grains of sodium chloride, an ionic compound under the microscope you will see that they have a cubic shape. The image below shows the structure of the sodium chloride lattice. The green spheres represent chloride ions (Cl-) and the blue spheres sodium ions (Na+).
If you were to count the ions in the model you would notice that
there are equal numbers of sodium ions, Na+, and
chloride ions, Cl-. This is simply because when the ions pack together they do in
such a way that the charges always
cancel out. The +1 charge on the sodium ion is cancelled out by the -1 charge on th
chloride ion. So the ratio of
the ions is 1:1. This simple ratio of ions enables you to calculate the empirical (simplest) formula for the compound.
So in the case of sodium chloride it is NaCl. Its ionic formula is Na+Cl-.
You will often see the lattice structure for ionic compounds shown in different ways. This is to try and help you visualize what the giant structure looks like. Opposite is shown the cubic lattice for sodium chloride (Cl- shown in green, Na+ is a yellow/green colour), but this time there are bonds shown between the ions. In reality there would be no gaps between the ions as they will pack together as closely as possible but it does allow you to view the interior of the structure which is not possible from the first model. Both model allow you to visualize how the ions are arranged in the structure.
None of the models for sodium chloride shown allow
you to spot immediately how the crystal structure is built. If
you pick out one sodium ion you should see that it has six immediate neighbours. One
chloride above it and one below,
one to its left and one to its right and one in front and one behind. This is shown opposite. The same is true if you
pick a chloride ion. Each sodium or chloride
ion has 6 immediate neighbours, the ions are described as 6 co-ordinate.
All ionic compounds have a giant lattice structure made up of ions. However not all ionic lattices have a face centred cubic structure like sodium chloride. Remember the ions will always try and pack together as closely as possible, but in a way that maximises the attraction between oppositely charged ions and reduces the repulsion between ions of a similar charge. However the different size of the ions in the compound will limit how closely they can pack together to maximise attractions and reduce repulsion. The image below shows ball and stick models for the structure of the calcium fluoride lattice. Clearly it is not cubic like the sodium chloride lattice because the ions are different in size and charge. In the models the red ions are Ca2+ and the blue ions are F-.