Atoms

Lesson

Part 1 — firing alpha particles at gold leaf, finding the nucleus, and Bohr’s rules for the electron.

In this lesson you will

Describe the Geiger–Marsden alpha-scattering experiment, its observations and Rutherford’s conclusions
Calculate the distance of closest approach of an alpha particle and use it to estimate the size of a nucleus
State Bohr’s postulates and derive the radius and speed of the electron in the nth orbit of hydrogen

Concept

Before the nucleus: Thomson’s model

J. J. Thomson pictured the atom as a sphere of positive charge spread evenly through its volume, with electrons embedded in it, like currants in a bun. A fast, heavy α particle (charge +2e) passing through such an atom meets only weak, spread-out forces.

Prediction: every α particle goes through a thin foil with a deflection of a small fraction of a degree.
Nothing in the model can turn an α particle back.

Table comparing the Thomson and Rutherford models of the atom: where the positive charge is and what each predicts for alpha particles passing through a thin foil
Model	Positive charge	Prediction for α particles
Thomson	Spread through the whole atom, electrons embedded in it	Only tiny deflections; none thrown back
Rutherford (1911)	Concentrated in a tiny central nucleus	Mostly straight through; a few large deflections, some thrown back

Diagram

The Geiger–Marsden experiment

A narrow beam of fast α particles from a radioactive source in a lead block falls on a very thin gold foil. A zinc sulphide screen viewed through a microscope turns round the foil; each α particle that strikes it makes a tiny flash (scintillation), and the flashes are counted at each scattering angle θ.

The whole apparatus is evacuated, so the α particles are not stopped or scattered by air.
Gold can be beaten very thin, so most α particles meet only one nucleus on the way through.

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