A model of vibrating atoms in a solid

Demonstration

This model shows atoms making up a crystal held together by spring-like forces.

Apparatus and Materials

• Atom model

Health & Safety and Technical Notes

Read our standard health & safety guidance

The model recommended is large and wobbly. The more rigid types often used in chemistry departments are not suitable.

Apparatus manufacturers may stock the version needed. The most useful versions are those where the springs are weak enough for the model to wobble like a jelly. However, it must be able to stand up on its own.

Alternately, a model may be made from foamed polystyrene spheres (or even the more massive golf balls) and weak steel springs. The springs have loops on each end and are intended for stretching experiments. Each one should be pre-stretched a little so the coils are separated. Slots are made in the spheres (a very hot domestic table knife blade works well) and the loops on the springs glued into slots.

A cubical array of 27 spheres will do.

Procedure

1. The model is used as a prop to support a discussion of the way in which the particles in a solid are held together by spring-like forces.
2. Show the model to the class and demonstrate how vibrating it will shake the individual atoms, but that they can retain their place in the overall pattern.

Teaching Notes

• For this purpose, this is a better model of a small crystal than one made of spheres glued together. It helps students to think about the part played by the forces holding atoms together.
• The model solid is 'heated' by shaking it. This represents energy transferred to the solid so that it is stored thermally. When the model solid is 'heated', the individual balls start to move. This leads to the idea that, in a solid at a given temperature the atoms may be vibrating. Energy stored thermally is due to these vibrations.
• Heat the solid more and the vibrations become more violent. There are two consequences.
• First, the structure becomes bigger/takes up more space. This helps students to understand thermal expansion.
• Second, if enough energy is transferred, the vibrations become so violent that the structure breaks up. Melting is thus explained. Or is this really a picture of sublimation (or change directly from solid to gaseous state)?
• If the outer layer of atoms are vibrating violently the vibration will be communicated to the next layer of atoms. This shows how energy might be conducted through a solid material. The vibrations themselves increase in amplitude when more energy is transferred to the model, but the frequency remains the same. You can imagine the atoms in a solid elbowing each other a little further apart as they vibrate more and more.
• At advanced level you would go into details of the balance between the short-range attractive forces between atoms and the very short-range repulsive forces between the same atoms. These forces must maintain the system in equilibrium, changing their values when atomic vibrations increase because those vibrations carry individual atoms to different distances where they experience different forces. As a result of those changes of forces, the whole array takes up a different length and strength, again in equilibrium. This is too complicated a story to tell at an introductory level.
• All scientific models have limits and they do not behave like the real thing. It is important to stress the limits of the models in use.

This experiment was safety-tested in March 2006