Overview
This lecture covers Rutherford's gold foil experiment, its setup, the results, and how it disproved the plum pudding model and led to the discovery of the atomic nucleus.
Background: Plum Pudding Model
- J.J. Thomson's "plum pudding" model described atoms as electrons embedded in a positively charged, low-density substance.
- Electrons were randomly scattered throughout the atom in this model.
Gold Foil Experiment Setup
- Conducted in 1909 by Geiger and Marsden under Rutherford's supervision.
- A radioactive element emitted alpha particles (helium nuclei, positively charged) from a lead box.
- Alpha particles were directed at a thin sheet of gold foil.
- Detector screens surrounded the foil to observe where particles traveled after striking the foil.
Expected Results Based on Plum Pudding Model
- All alpha particles should pass straight through the gold foil with minimal deflection.
- Compared to firing cannonballs through tissue paper: almost no resistance expected.
Actual Results
- Most alpha particles passed through the foil as expected.
- Some alpha particles deflected at various angles.
- A few alpha particles bounced straight back toward the source.
Rutherford's Conclusions
- Atoms have a small, dense, positively charged center called the nucleus.
- The rest of the atom is mostly empty space, allowing most alpha particles to pass through.
- Deflections occur when alpha particles encounter the dense nucleus; direct hits cause particles to bounce back.
Key Terms & Definitions
- Alpha particle — Helium nucleus (2 protons, 2 neutrons) with a positive charge.
- Nucleus — Dense, positively charged center of the atom.
- Plum pudding model — Early atomic theory where electrons are scattered in a positively charged substance.
Action Items / Next Steps
- Review Rutherford’s model versus the plum pudding model.
- Prepare for questions about the experiment setup and its implications for atomic structure.