Shooting at gold foil
Geiger and Marsden's alpha-particle scattering.
Aim at a thin sheet
Imagine a narrow beam of tiny positively charged particles, called alpha particles, aimed at an extremely thin sheet of gold. In Thomson's model the positive charge is spread out evenly in the atom.
What did Thomson's model expect the alpha particles to do?
What this lesson covers
The idea
When positively charged alpha particles are aimed at a very thin gold foil, most pass through undeflected, some are sharply deflected and a few bounce back; Thomson's model cannot explain this scattering.
Aim at a thin sheet
Imagine a narrow beam of tiny positively charged particles, called alpha particles, aimed at an extremely thin sheet of gold. In Thomson's model the positive charge is spread out evenly in the atom.
What did Thomson's model expect the alpha particles to do?
- Pass straight through or be deflected only slightly
- Bounce straight back from the gold
- Be stopped inside the gold
The gold foil experiment
Press Next step to see what was expected and what happened.
A surprising result
Most alpha particles went straight through, but some were sharply deflected and a few bounced back. Thomson's evenly spread positive charge could not explain that.
When positively charged alpha particles are aimed at a very thin gold foil, most pass through undeflected, some are sharply deflected and a few bounce back. Thomson's model cannot explain this scattering.
Notes
Alpha particles on thin gold foil: most pass through, some are sharply deflected, a few bounce back. Thomson's model cannot explain this scattering.
Check yourself
What happened to most of the alpha particles?
What did Thomson's model lead scientists to expect?
What is the deflection of alpha particles from their straight path called?
Alpha particles are …
- They bounced back. Only a few alpha particles bounced back.
- They passed through the gold foil undeflected — correct. Yes!
- They were sharply deflected. Only some alpha particles were sharply deflected.
- Most alpha particles would bounce back. With positive charge spread evenly, strong bounces were not expected.
- The alpha particles would stick to the foil. Thomson's model expected them to pass straight through or be deflected only slightly.
- The alpha particles would pass straight through or be deflected only slightly — correct. Yes!
- Scattering — correct. Yes!
- Radioactivity. Radioactivity is the emission of radiation by certain elements.
- Stability. Stability is about atoms staying intact and not collapsing.
- negatively charged, like cathode rays. Cathode rays are streams of electrons. Alpha particles carry positive charge.
- positively charged particles from certain radioactive elements — correct. Yes!
- particles with no charge. Alpha particles are tiny, positively charged particles.