Describe the Geiger-Marsden alpha-particle scattering experiment. Write its main observations, the conclusions drawn from them, and draw the graph of the number of scattered particles against scattering angle.

1. 1 · State the aim in one line. To probe the inside of the atom by firing alpha particles at a thin gold foil and counting how they scatter.
2. 2 · Draw and describe the set-up. [Diagram: lead block with ²¹⁴Bi source, lead slits, thin gold foil, rotatable ZnS screen and microscope, angle θ marked, vacuum]
   • 5.5 MeV alpha particles, collimated into a beam
   • Gold foil about 2.1 × 10⁻⁷ m thick
   • Flashes counted at different θ
3. 3 · Write the observations.
   • Most pass undeflected
   • About 0.14% are deflected by more than 1°
   • About 1 in 8000 are deflected by more than 90°
4. 4 · Write the conclusions.
   • The atom is mostly empty space
   • All positive charge and nearly all mass are in a tiny nucleus, about 10⁻¹⁵ to 10⁻¹⁴ m
   • Electrons lie outside the nucleus
5. 5 · Draw the graph. [Graph: N(θ) on a log scale against θ from 0° to 180°; steeply falling curve with experimental points]
6. 6 · Real life. Ion-beam scattering is used today to check thin films in solar cells and chips.
7. 7 · Close in one line. So, the atom has a tiny, dense, positively charged nucleus at its centre.

Scoring tip: Write the numbers 0.14% and 1 in 8000; examiners look for them. Mark θ on both the set-up and the graph.