Rutherford Scattering Experiment (1909)
One of the most important experiments in atomic physics, carried out by Hans Geiger and Ernest Marsden under the direction of Ernest Rutherford.
1. Background
Before this, the "Plum Pudding" model (J.J. Thomson) was accepted: atoms were spheres of positive charge with electrons embedded like raisins. If correct, alpha particles should pass through with almost no deflection.
2. Apparatus
- Radioactive Source: Emits high-speed, positively charged alpha particles.
- Lead Box: Absorbs radiation, creating a narrow beam.
- Thin Gold Foil: A very thin sheet of gold was placed in the path of
alpha particles.
Thickness: About 1000 atoms thick (so thin that scientists could practically count the atoms). - Fluorescent Screen: Coated with Zinc Sulfide; flashes (scintillations) when hit.
The experiment produced three main observations:
3. Observations & Conclusions
About 99% of alpha particles went through the foil without deflection.
Conclusion: The atom is mostly empty space.
A small fraction of particles changed direction by small angles.
Conclusion: Positive charge exists in the atom and repels alpha particles.
About 1 in 20,000 particles bounced back almost 180°. This was extremely surprising.
Conclusion: A very small, very dense region exists inside the atom.
"It was as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you." — Ernest Rutherford
4. Rutherford’s Atomic Model
From the experiment, Rutherford proposed the nuclear model of the atom.
Main features:
- Tiny central nucleus: Contains most of the mass and is positively charged.
- Electrons: Revolve around the nucleus.
- Empty space: Most of the atom is empty space.