Polystyrene sphere crystal models

Class practical

This shows ready-made models, giving an opportunity to link the shapes of real crystals with an underlying structure.

Apparatus and Materials

• Adhesive
• Polystyrene spheres

Health & Safety and Technical Notes

If it is necessary to use solvents to prepare an adhesive, remember that some students may try to acquire them for sniffing!

Read our standard health & safety guidance

Prepare the models some time before the lesson by joining spheres together with a suitable adhesive (e.g. Copydex or a PVA adhesive). If conventional adhesives do not work, dissolve some sheet-foamed polystyrene (or one of the spheres) in propanone or, better, in amyl acetate. The adhesive should be very thick in consistency. Apply it with a wood splint to the spheres at the points of contact. Then press them firmly together. It will take 24 hours for the glue to harden.

You may prefer to use cocktail sticks or double-ended screws but gluing is easier.

Simple cubic lattice: Make four layers and place them one on top of the other. The layers can be fixed permanently together, left separate, or tied together with cotton.

Body-centred cubic lattice: This can be built up as illustrated. Put a second layer of nine spheres on top of the first layer of sixteen as shown. Put a layer of sixteen (already stuck together) similar to the first, on top of this layer. Then repeat the process.

Alternately, build a pyramid. Starting with a 4-sphere x 4-sphere base, then 9 in the second layer, 4 in the third, and 1 in the last.

Or make a large number of 4 x 4 layers and demonstrate both simple cubic and body-centred cubic by arranging the layers appropriately.

Hexagonal close-packed lattice: Build a close-packed, single-layer structure, as shown, with each sphere touching six others. Place a second layer, similarly constructed, on top of the first. Displace it slightly so that the spheres of the second layer fit into depressions in the first. Further layers can then be placed on top.

Procedure

The crystal models can be added to other exhibits of crystals.

Teaching Notes

• Although the different packings should be shown, it would probably be wise not to attempt to enlarge on detail and certainly not to give the long names for the different crystalline structures, except with advanced students.
• You might merely point out that these models appear to agree with the shapes and behaviour of crystals.

This experiment was safety-tested in June 2004