Rutherford's Model of the Atom

By the end of this activity, students should

1. Be familiar with the nature of Rutherford's scattering experiment.
2. Be able to articulate that electrical forces, rather than gravitational, are the dominant forces at the atomic scale.
3. Recognize the specific elements the experiment that cause the deflection of alpha particles in Rutherford's scattering experiment (also recognizing the elements that do not cause deflection).
4. Be able to predict general scattering patterns based on given information of alpha particle kinetic energies. This is an optional, more subtle goal.

• Educational level: Undergraduate modern physics
• Setting: Classroom lecture
• Time required: 15 minutes +
• Special equipment: Java capable computer, digital projector, access to internet
• Pre-requisite knowledge: Nature of charges, gravitational interaction, kinetic energy

SETUP AND INTRODUCTION
Materials needed:

• Computer with internet accessibility
• Digital projector
• Capability of running Java applets
• Clicker software and hardware setup (optional)

In class

1. Load the applet Rutherford Scattering
2. Locate the "Pause" button at the bottom of the applet
3. Press the power button on the alpha gun and then quickly press "Pause" (Ideally, you would like students to see that alpha particles are approaching the nucleus without revealing the interaction.)
4. Ask the first think-pair-share question:
   A stream of alpha particles is headed toward the nucleus of an atom. What is going to happen?
   While students are working with their partners, ask them to consider the following:
   • The charge of protons, electrons, neutrons, and an alpha particle
   • The mass of protons, electrons, neutrons, and an alpha particle
   • The kinetic energy of the alpha particles
5. Ask a few student pairs to share their ideas with the class.
6. Press the "Play" button at the bottom of the applet to reveal the interaction. At this point, select the "Show traces" checkbox so that the paths of the alpha particles will be clear. Leave this feature on for the remainder of the activity.
7. Now that students have observed the interaction, ask the following question:
   What do you think is the cause for this observed interaction between the nucleus and the alpha particles?
   Make sure students are given ample time to formulate ideas.
8. From here on, it is up to the instructor to decide the length of the remainder of the activity. It is strongly recommended that at least some of the ideas students present are tested within the applet. Ideas easily tested within the applet include:
   • Changing the energy of the alpha particles
   • Changing the number of protons in the nucleus
   • Changing the number of neutrons in the nucleus

This activity could be used to introduce Rutherford's experiment or adapted to follow up on an assigned reading.

The applet used in this activity is very intuitive. Not mentioned above is the "Plum Pudding Atom" model available within the applet. It can be selected by clicking on the separate tab at the top of the applet. This would be prudent to show as a demonstration of what an atom would look like for "no interaction" responses to the first think-pair-share question.

This activity is easily adapted for use with a clicker system. For convenience, a template has been created in PowerPoint for use with the H-ITT clicker system:Clicker Questions (PowerPoint 79kB Jul25 07) Asking the class questions in this way is still compatible with the think-pair-share strategy of instruction. In fact, with proper management, clicker use can increase the level of discussion and student participation.

If clicker questions are used during the activity, post-testing the same questions could provide a quick assessment. A sample of additional relevant questions is available in this document:Sample evaluation questions (Microsoft Word 26kB Jul25 07).

The Rutherford Scattering Java applet is managed by PhET. One question in the sample assessment questions is taken from Randall D. Knight's Physics for Scientists and Engineers with Modern Physics, 2004.