Journey Inside the Atom · Lesson 3 of 14
Testing Thomson's Model: The Gold Foil Experiment
“Alpha-particle scattering supplies the evidence that leads to a nuclear atom”
• Describe the gold foil experiment. • Explain alpha particles within the chapter's treatment. • Distinguish predicted and observed scattering. • Define scattering. • Explain why the observations contradict Thomson's model.
A scientific model must survive experimental testing. Thomson's model spread positive charge throughout the atom. If that picture were correct, a positively charged particle moving through a thin foil should not meet a tiny region of strongly concentrated positive charge.
Testing Thomson's Model: The Gold Foil Experiment
The Experimental Setup
Geiger and Marsden, working under Ernest Rutherford, directed a narrow beam of alpha particles at an extremely thin gold foil. Alpha particles are positively charged particles emitted by certain radioactive elements. Within this chapter, an alpha particle is treated as a helium nucleus containing two protons and two neutrons.
A positively charged particle emitted by certain radioactive elements; it is a helium nucleus containing two protons and two neutrons.
What Thomson's Model Predicted
Because Thomson's positive charge was spread throughout a large volume, alpha particles were expected to pass through the foil almost straight or suffer only small deflections. There was no tiny massive positive centre in the model that could strongly repel an alpha particle.
What Was Observed
Most alpha particles passed through undeflected. Some were deflected through noticeable angles. A very small number were deflected through very large angles, and a few even bounced back. The change of direction from the original path is called scattering.
Deflection of particles from their original straight-line path.
| Observation | Structural clue |
|---|---|
| Most particles passed straight through | Most of the atom does not contain dense matter that strongly interacts with the alpha particles. |
| Some were deflected | Positive charge is present and repels the positive alpha particles. |
| Very few were strongly deflected or bounced back | A large positive charge and much of the mass must be concentrated in a tiny region. |
Why Thomson's Model Failed
A diffuse positive sphere could not produce the strong repulsion required to send a fast positive alpha particle back toward the source. The rare backward paths therefore showed that positive charge could not be spread uniformly through the entire atom.
Connect each observation to a structural conclusion. Straight paths point to empty space; deflections point to positive charge; rare large deflections point to a tiny dense concentration of positive charge and mass.
Problem
What does a rare backward deflection tell us?
- 1.An alpha particle is positively charged.
- 2.A backward deflection requires strong repulsion.
- 3.Spread-out positive charge would produce only weak deflection.
- 4.The alpha particle must occasionally approach a very small strongly positive region.
- 5.Because the event is rare, this region occupies only a tiny part of the atom.
Quiz
Which description best matches Alpha Particle?
Which description best matches Scattering?
Which term matches this description: A positively charged particle emitted by certain radioactive elements; it is a helium nucleus containing two protons and two neutrons.
Which term matches this description: Deflection of particles from their original straight-line path.
Which statement is a key takeaway from this lesson?
Practice Problems
- Predict qualitatively what might change if the gold foil were thicker.
- Why is a backward deflection especially damaging to Thomson's model?
- Why is the observation that most particles pass straight through also important?
- What might change if negatively charged particles were used instead of alpha particles?
Key Takeaways
• Rutherford's gold foil experiment used alpha-particle scattering to test the structure of atoms. • Most alpha particles passed straight through, showing that most of an atom is empty space. • A small number were strongly deflected, showing that positive charge is concentrated in a tiny region. • Rare backward deflections provided strong evidence for a dense central nucleus.