1. You need to know: The structure of an atom The charge of a nucleus The structure of an alpha particle + + 

2. Lesson Outcomes What was the plum pudding model? GRADE C How was the plum pudding model disproved? GRADE B Describe Rutherford, Geiger and Marsden’s experiment. GRADE B Explain their findings. GRADE A

3. Rutherford’s Alpha Scattering Experiment The is one of the most important experiments in the history of science. Professor Rutherford wanted to see what happened to alpha particles when they collided with atoms. He asked two of his students (Hans Geiger and Ernest Marsden) to carry out the experiment.

4. Shield Source of alpha particles Very thin gold foil Vacuum (air pumped out) Moveable Microscope Zinc sulphide screen (glows when hit by alpha particles) Rutherford Experiment: the set up Geiger and Marsden counted the tiny green flashes in the microscope produced when alpha particles hit the screen. Both Geiger and Marsden described this as one of the most difficult and boring experiments they’d ever had to do.

5. Geiger and Marsden placed the microscope as shown. As expected, most of the alpha particles went straight through the foil. A few alpha particles were scattered by angles less than 90º, also as expected.

6. Why didn’t you put the microscope behind the gold foil? Rutherford Geiger Marsden There’s no point! There is absolutely nothing inside an atom that could reflect an alpha particle! It would be a complete and utter waste of time!

7. Under protest, Geiger and Marsden placed the microscope behind the gold leaf. Much to their surprise, a very small number of alpha particles (about 1 in 8000) bounced off the gold atoms! They handed the results to Professor Rutherford who now had to explain what was going on.

8. Up until Rutherford’s experiment, it was thought that atoms were like a ‘plum pudding’ – the electrons were negative “plums” embedded in a ball of positive pudding. But, this is what he found. What do you think he deduced about the structure of an atom from these results?

9. Rutherford’s Alpha Scattering

10. The top scientific honour in the world is the Nobel Prize. Ernest Rutherford was awarded the Nobel Prize in 1908 for discovering the atomic nucleus. (He deserved it – it was his calculations based on the data that measured the size of the nucleus. Geiger and Marsden were given full credit in the published scientific paper.) However, he was slightly disappointed because he was given the Nobel Prize for Chemistry instead of Physics Rutherford All science is either Physics or stamp collecting.

11. Rutherford’s Model (Nuclear Model) The electrons ‘live’ in specific energy levels. An electron will move up to a higher energy level if it is given energy. An electron will move down to a lower energy level if it loses energy.

12. Lesson Outcomes What was the plum pudding model? GRADE C How was the plum pudding model disproved? GRADE B Describe Rutherford, Geiger and Marsden’s experiment. GRADE B Explain their findings. GRADE A

13. The protons inside a nucleus repel each other because they each have a positive charge. It takes an incredibly strong force to hold the nucleus together. This incredibly strong force is called the strong nuclear force. Unlike the electromagnetic force, it has a very short range. How do the protons manage to stay in the nucleus?