Describe Thomson's model of the atom with a labelled sketch, and give one limitation of it.

1. 1 · Basis. Atoms are neutral but contain negative electrons, so they must also hold an equal positive charge.
2. 2 · The model. The atom is a sphere of evenly spread positive charge with electrons embedded in it.
3. 3 · Sketch. A large circle with + signs spread through it and small − dots for electrons, with labels.
4. 4 · Everyday picture. It is like a watermelon: pulp for the positive charge, seeds for the electrons.
5. 5 · Limitation. It could not explain why a few alpha particles bounced back from gold foil; it had no nucleus.
6. 6 · Close. So it was replaced by Rutherford's nuclear model.

Scoring tip: A third of the marks are for the limitation; link it to the alpha scattering experiment.