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

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

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.

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. Moveable Microscope Zinc sulfide screen (glows when hit by alpha particles) Shield Source of alpha particles Vacuum (air pumped out) Very thin gold foil

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.

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

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.

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?

Rutherford’s Alpha Scattering

All science is either Physics or stamp collecting. The top scientific honor in the world is the Nobel Prize. Ernest Rutherford was awarded the Nobel Prize in 1908 for diskovering 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.) All science is either Physics or stamp collecting. However, he was slightly disappointed because he was given the Nobel Prize for Chemistry instead of Physics Rutherford

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.

Simulation of Rutherford’s Scattering Experiment To see and work with a simulation of this experiment please do the following: Go to: http://phet.colorado.edu/en/simulation/rutherford-scattering Click on the Run Now button (or download if you would like) Turn on the electron gun by clicking on the number in the red circle on the left You can change the energy of the alpha particles coming out of the gun by using the slider on the right. You can also change the size of the atoms in the chamber by using the appropriate slider on the right. The next slide offers a couple of questions related to the simulation.

Questions from the Simulation A couple of questions: Do the nuclei (plural of nucleus) affect the paths of the alpha particles more or less when the alpha particles are at higher energies? Keep the number of neutrons fixed at some number and increase the number of protons (while keeping the alpha particle energy constant). As the number of protons increases, what can you say about the approach distance of the alpha particles to the nuclei – does it go up, down, or remain the same? Keep the number of protons fixed at some number and increase the number of neutrons (while keeping the alpha particle energy constant). As the number of neutrons increases, what can you say about the approach distance of the alpha particles to the nuclei – does it go up, down, or remain the same?

Answers to Questions from the Simulation Do the nuclei (plural of nucleus) affect the paths of the alpha particles more or less when the alpha particles are at higher energies? The path of the alpha particles is affected less when they are at higher energy. This is similar to trying to change the direction of a moving car – the more slowly it is moving, the easier it is to change. Keep the number of neutrons fixed at some number and increase the number of protons (while keeping the alpha particle energy constant). As the number of protons increases, what can you say about the approach distance of the alpha particles to the nuclei – does it go up, down, or remain the same? As the number of protons changes, the alpha particles (which are also positively charged) do not approach as closely. Like charges repel each other. Keep the number of protons fixed at some number and increase the number of neutrons (while keeping the alpha particle energy constant). As the number of neutrons increases, what can you say about the approach distance of the alpha particles to the nuclei – does it go up, down, or remain the same? As the number of neutrons changes, the alpha particle paths do not change much at all. Neutrons, as we will see shortly, carry no charge and thus have no electrical interaction with alpha particles.

Conclusions from Rutherford’s Scattering Experiment Most of the atom is empty space – most of the alpha particles go straight through the foil There are concentrated regions of positive charge with high mass in the atom – some of the large positively charged alpha particles bounce nearly straight back. Rutherford proposed a nuclear theory of the atom where the protons are localized in a small core called the nucleus and the electrons are outside of the nucleus. See image to the right. Image from www.clemson.edu/

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