Previously, the ‘plum pudding‘ model of an atom was believed; atoms were spheres (+ve) with tiny electrons (-ve) inside.
If this was true, the alpha particles would’ve gone through the sheet of gold.
To test this theory, Geiger and Marsden set up an experiment:
- A beam of alpha particles was aimed at very thin gold foil
- Passage through foil was detected
- Scientists expected particles to pass straight through. HOWEVER
Some alpha particles passed through, but others deflected the foil at different angles, and some came straight back.
- Realised that positively charged alpha particles were being repelled and deflected by a tiny concentration of positive charge in the atom.
Alpha particles travelled straight through the electron shells without changing direction.
If alpha particle gets close to nucleus (+ve), it repels and changes direction.
The closer it got to the nucleus, the more it changed direction.
Replaced with Rutherford’s nuclear model of the atom.