# Crystal models made of marbles

##### Class practical

Students pile marbles into a cardboard tray and so find a convenient way of packing them together.

#### Apparatus and materials

*For each student group*

Marbles, 1.5cm diameter, 55

Card, thin, approx 13cm square

#### Health & Safety and Technical notes

A lively class will need careful instruction to avoid spilling marbles all over the floor.

Improvise the trays on which the marbles are to be stacked from the sheets of card. These are marked with lines about 2 cm in from the edges as shown in the diagram. Most important is that when the edges are folded up along these lines, a row of 5 of your marbles can snugly but comfortably fit between them. Make cuts as shown, and fold and staple the edges to form a square tray.

The base of the tray must hold a layer of 25 marbles. Marbles tend to vary a bit in size.

#### Procedure

Students pour in marbles to form a layer of 25 marbles. On top of this they add layers of 16, 9, 4 and 1 marbles to form a pyramid.

####

Teaching notes

Students should notice the shape of the model crystal they are building and the angles between its faces. (They may need reminding that their model is of a tiny number of particles/atoms compared with that making up a real crystal.)

You could ask students if they can identify real crystals with the same shape or angles between faces as their model. Alum would be a good example.

You can draw students' attention to the fact that even if a marble is (carefully!) removed, the shape of the 'crystal' remains.

*This experiment was safety-checked in September 2004*