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+).

Formula of an ionic compound

If you were to count the ions in the model opposite 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 charge always cancel out. The +1 charge on the sodium ion is cancelled out by the -1 charge on the 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 giant 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.

Co-ordination numbers

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 or are 6 co-ordinate. 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-.

What will be the empirical formula for calcium fluoride?

• calcium is a group II metal so has a 2+ charge. Ca2+
• fluoride is a group 7 non-metal, so has a 1- charge, F-
• We need to balance the charges, so will need 2 x F- ions to balance the 2+ charge on the calcium ion.
• So empirical formula is CaF2

Group ions

Some ions are compounds and contain more than one type of ion. You will have met these ions in the past but perhaps not realised they were group ions. For example the sulfate ion, SO42-, ammonium ion NH4+ and hydroxide ion OH- are all compounds and contain more than one element. These compounds are often called polyatomic ions or group ions. They are shown below: When working out the formula of compounds containing these ions we just use the same rules as before.

What will be the empirical formula for sodium sulfate?

• sodium is a group I metal so has a 1+ charge. Na+
• sulfate is a group ion , with a charge of 2-
• We need to balance the charges, so will need 2 x Na+ ions to balance the 2- charge on the sulfate ion.
• So empirical formula is Na2SO4

Key Points

• Ionic compounds consist of a giant lattice structure composed of positively charged metal ions and negatively charged non-metal ions.
• An ionic bond is the attraction between a positively charged metal ion and a negatively charged non-metal ion.
• The empirical formula for an ionic compound will give you the ratio of the metal to non-metal ions in the structure.
• A crystalline structure is an ordered one, this means it is built up in a regular way e.g. you could for example describe a brick wall as crystalline, as the bricks are arranged in a regular and repeating pattern. If you knock the wall down and simply have a pile of bricks then this could be described as a non-crystalline arrangement or amorphous. Ionic compound have a giant crystalline lattice structure.