1. Structure and bonding
page 5
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Photo of stacked fruit
Picture 1.7 These oranges form a fairly good hexagonal close packed structure when they are stacked.
The crystal structure of metals
When we look at the surface of a metal, it is often possible to see crystals. The crystals that we can see reflect the symmetrical arrangement of positive metal ions in the underlying structure that we can’t see.

The positive ions in a metal are packed closely together so that the gaps between them are kept to a minimum. There are three different ways in which the ions can be packed together:

Coordination numbers and unit cells
In the different arrangements the positive ions may have different co-ordination numbers. This is the number of close contacts with neighbouring positive ions each metal ion has. The ions may also take up different amounts of space and their unit cells are different.
Interactive graphic of HCP
Picture 1.8a HCP packing. Use the toll overs in the text. The colours signify the different layer positions.
Hexagonal close packing
In the hexagonal close packed system, 74% of space is occupied by the positive ions. The structure is made up of different alternating layers of the kind ABABA. The positive ions have a co-ordination number of 12. They are touching 12 other ions - 6 in their own layer and 3 in each of the adjacent layers.

You will get hexagonal close packing if you stack marbles in the following way. Fill the bottom of a tray with marbles. They will arrange themselves in hexagons – any marble will be touching six other marbles.

Now make another layer. The marbles in the new layer will fall in the gaps between the bottom marbles. Now start making a third layer. However, you have to make sure that the marbles in the third layer lie above the marbles in the bottom layer.

We call this arrangement ABAB (because the layers' positions alternate).

Interactive graphic of HCP
Picture 1.9a Use the rollovers in the text to build up HCP packing. Note, the colours are to signify the different layers positions.
Interactive graphic of FCC
Picture 1.8b FCC packing. Use the toll overs in the text. The colours signify the different layer positions.
Face centred cubic close packing
In the face centred cubic close packed system, 74% of the space is again occupied by the positive ions. Again, the bottom layer is made of hexagons. The second layer fits in the gaps as before. However, the third layer is different.

The ions in the third layer layer lie above gaps in both of the previous layers. We say that the layers are arranged as ABCABC. Compare this with ABAB where the third layer lines up with the first. The positive ions have a co-ordination number of 12 as before.

This arrangement is the same as a face centred cubic structure. The main diagonal of the cube runs at right angles to the layers. We need to add a single ion into a layer beneath our bottom layer to see the face centred cube. Notice it is made of one ion in this new layer, one ion on the top layer and three ions in each of the sandwiched layer (arranged in triangles facing opposite ways).

Interactive graphic of FCC
Picture 1.9b Use the rollovers in the text to build up HCP packing. Note, the colours are to signify the different layers positions.
Interactive graphic of BCC
Picture 1.10 Model of body centred cubic unit cell.
Body centred cubic packing
In the less closely packed body centred cubic system, positive ions occupy only 68% of the space. The structure is made up of cubes with a positive ion at each of the corners and with one ion in the centre of the cube. The positive ions this time have a co-ordination number of 8.

This is not a close packed structure and you won’t be able to build it from marbles (because they will always fall into gaps).

Question 1-4.
a) In what three ways may the crystal structures of metals differ from each other?

b) In which crystal packing system are there three different alternating layers?


Summary                   Close
  • metals have a crystal structure which reflects the packing of the ions in the lattice
  • there are two types of close packing: hexagonal and face centred cubic
  • some metals have a body centred cubic structure; this is not closely packed
  • the tightness of the packing is measured by the co-ordination number