Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

CHEMISTRY · Stage 8 · Chapter 7: Inside Atoms

Chemistry 7.3 Finding the nucleus — Rutherford’s Gold Foil Experiment

5 min read 6 parts Lesson 3 of 6

0%read
Lesson summary

7.3 Finding the nucleus — Rutherford's Gold Foil Experiment — Alhussan International Academy — Chemistry (Stages 7–9) notes: Describe the gold foil experiment and explain what it showed about the structure of the atom and how scientists build on each other's work.Background — Tho…

  • Prediction
  • Collaboration
  • Experimental design
  • Rate of bounce back
  • Empty space
  • Explaining the results

Part 1 of 6Testing the plum pudding model

1Background1 point
— Thomson's model had good evidence for negative electrons, but no evidence for their arrangement in a positive sphere.
  • Ernest Rutherford lived in New Zealand until he was 23 and moved to England in 1895 to study under Thomson.
2Prediction1 point
— if the plum pudding model is correct, most positive particles fired at gold foil go straight through.
  • A few would pass close to negative electrons and change direction slightly.
3Collaboration
— Rutherford worked with Hans Geiger and Ernest Marsden.

Part 2 of 6The gold foil experiment (1909)

  1. A radioactive source emits a narrow beam of alpha particles, which are positively charged.
  2. The beam is fired at a very thin sheet of gold foil, only a few atoms thick.
  3. A fluorescent screen around the foil flashes when an alpha particle hits it.
  4. The positions of the flashes are recorded to show where the particles went.
  • Experimental design — gold can be hammered into very thin sheets because it is malleable.
    • A thick foil would scatter particles many times so single events could not be interpreted.
    • The apparatus is in a vacuum so air does not absorb the alpha particles.

Part 3 of 6Observations and conclusions

ObservationConclusion
Most particles passed straight throughThe atom is mostly empty space
Some particles were deflectedThe nucleus has a positive charge and repels positive alpha particles
A few bounced backThe positive charge and most mass are concentrated in a tiny, dense nucleus
  • Rate of bounce back — around one in every 10 000 particles bounced backwards off the foil.
    • Rutherford said this was as incredible as firing an artillery shell at tissue paper and having it come back.
  • Result — the prediction based on the plum pudding model was wrong, so the model must be wrong.

Part 4 of 6Rutherford's nuclear model (1911)

1Nucleus
— atoms have a central, positively charged nucleus containing most of the mass.
2Empty space1 point
— the nucleus is surrounded by empty space in which electrons move.
  • The nucleus is much smaller than the atom, about 10 000 to 100 000 times smaller.
3Explaining the results1 point
— particles that went straight through passed through the empty space between nuclei.
  • The particle that bounced back hit a nucleus.

Part 5 of 6Inside the nucleus

  • Protons — Rutherford fired positive particles into the air and made tiny new positive particles from the nuclei of nitrogen atoms, which were protons.
  • Neutrons — the next year he suggested the nucleus also holds particles with mass but no charge.
    • By 1932 James Chadwick showed that a nucleus contains neutrons as well as protons.
  • Model today — positive protons and neutral neutrons in the nucleus, with negative electrons orbiting outside.

Part 6 of 6How scientists build on each other's work

  • Publication — Rutherford published a paper describing his model and the problems with the plum pudding model.
  • Acceptance — other scientists did not accept it at first, but over time developed it further.
  • Bohr — in 1913 Niels Bohr discovered that electrons move in fixed orbits (shells or energy levels), explaining why atoms do not collapse.
  • Collaboration — Marie Curie and Rutherford organised a conference in 1913 for scientists to discuss models of atoms.
  • Scientists involved — Dalton, Thomson, Rutherford, Bohr, Chadwick and Geiger/Marsden each built on earlier work.
    • Evidence that does not fit a model must be taken seriously, and sharing results lets others check them.

Last updated: