1. Hans Geiger Ernest Marsden

2. Hans Geiger Born in Germany on September 30 th 1882 Studied Physics in Munich before moving to Manchester Studied mathematical relationship between the amount of alpha scattering and atomic weight He invented the Geiger Counter in 1911 Died in Berlin, on September 24 th 1945

3. Ernest Marsden … was born in Lancashire in 1888. Like Rutheford (who he was later to work with) he won a scholarship to attend grammar school. He later gained entry to Manchester University He retired in 1954 but still served on many committees until his death in 1970

4. The Experiment: The source produces alpha particles which are made into a beam and directed at a gold foil. The particles scatter off the foil and are detected when they hit the screen

5. Results/Conclusions: The majority of the alpha particles passed through the gold foil but some were scattered at very large angles If Thomson’s model of the atom was correct this would have been impossible: there was no force to make this happen as the charge was evenly distributed Geiger and Marsden concluded that Thomson’s model of the atom was wrong

6. The New Theory: Ernest Rutheford concluded that the atom must have a tiny nucleus at the centre which was positively charged. In this model charge was no longer evenly distributed around the atom, although the atom itself was uncharged Therefore positively charged alpha particles would be repelled away from the positively charged nucleus, sometimes directly back at the alpha source

7. Old model of nucleus (plum pudding model) New model of nucleus (the Bohr model) Sphere of positive charge Electrons

8. How this theory was reached: Most of the alpha particles scattered only a few degrees or not at all – a small force must have been acting on them However some particles scattered a lot – a large force must have acted on them. The only force this could have been was positive charges repelling The alpha particles experiencing greater forces than the others must have been nearer to the charge than the rest

9. The atoms positive charge must be concentrated around a small area (the nucleus) with a relatively large distance between one nucleus and another The alpha particles passing through the gold far from a nucleus are unaffected by the tiny force, those near the nucleus are scattered, some by large amounts. Reaching the theory (continued):

10. Gold nucleus. ( + )

11. Quick Quiz: Q. What type of ionising radiation was used in the experiment? A.Alpha (α) Q. Who were Geiger and Marsden working for? A. Ernest Rutheford Q. What was used to scatted the alpha particles? A. Gold foil Q. What did Rutheford say about the alpha particles which were deflected back to the source ‘It was as if you fired a 15in shell at a piece of tissue paper and it came back and hit you!’