Atoms

Lesson

Part 1 — firing alpha particles at gold foil revealed that almost all the mass of an atom sits in a tiny nucleus.

In this lesson you will

Describe the Geiger–Marsden alpha-scattering experiment and its observations
Explain how the observations led to Rutherford’s nuclear model
Calculate the distance of closest approach and relate impact parameter to deflection

Concept

Thomson’s model

Thomson (1898): positive charge spread uniformly through the whole atom, with electrons embedded in it like seeds in a watermelon.

The atom is electrically neutral.
Prediction for fast alpha particles: the spread-out charge exerts only weak forces, so they should pass through with very small deflections.

Diagram

The Geiger–Marsden experiment

A narrow beam of alpha particles (about 5.5 MeV, from a radioactive source) falls on a thin gold foil (about 0.21 μm thick). A zinc sulphide screen viewed through a microscope flashes where each particle lands; it can be turned to count particles scattered through any angle θ.

The whole apparatus is in vacuum.
Gold is used because it can be beaten into very thin foils, so most particles meet only one nucleus.

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