Rutherford's Gold Foil Scattering Experiment

In 1911, Ernest Rutherford directed alpha particles at a thin gold foil. Most particles passed straight through, but a small fraction were deflected at large angles — some even bounced back.

This result was incompatible with Thomson's "plum pudding" model. It proved that atoms have a tiny, dense, positively charged nucleus surrounded by mostly empty space.

The scattering pattern follows the Rutherford scattering formula, where the probability of scattering at angle θ is proportional to:

Observation 1: Most alpha particles pass through — the atom is mostly empty space.

Observation 2: Some particles deflect at large angles — a concentrated positive charge (nucleus) exists.

Observation 3: Very few particles bounce back — the nucleus is very small but very massive.

Limitation 1: Rutherford's model cannot explain why orbiting electrons don't radiate energy and spiral into the nucleus (classical electrodynamics predicts they should).

Limitation 2: It cannot explain the discrete line spectra of elements — there is no mechanism for quantised energy levels.

Legacy: Scattering experiments remain fundamental in particle physics. Modern accelerators (e.g. CERN's LHC) use the same principle — firing high-energy particles to probe subatomic structure.