Describe Rutherford's alpha scattering experiment with a labelled diagram. What did it show, and what are two drawbacks of his model?

1. 1 · Set-up. A narrow beam of alpha particles from a radioactive source is aimed at a very thin gold foil, with a fluorescent screen around it.
2. 2 · Diagram. Source in a lead block, beam, gold foil, circular screen, and paths: most straight, some bent, a few turned back.
3. 3 · Observations.
   • Most pass straight through
   • Some turn through small angles
   • A few turn through large angles
   • About 1 in 12 000 bounce back
4. 4 · Conclusions. The atom is mostly empty space; all its positive charge and nearly all its mass sit in a tiny, dense nucleus.
5. 5 · The model. Electrons move around the nucleus, and the atom is neutral.
6. 6 · Drawbacks.
   • Cannot explain stability: a circling electron should spiral into the nucleus
   • Does not give the arrangement of electrons
   • Cannot explain why atoms are heavier than their protons alone
7. 7 · Close. Bohr's model removed the first two drawbacks, and the neutron removed the third.

Scoring tip: Pair each observation with its conclusion; the marking scheme gives a mark for each pair.