In the Classroom edited by
Tested Demonstrations
Ed Vitz Kutztown University Kutztown, PA 19530
Turning Plastic into Gold: An Analogy To Demonstrate the Rutherford Gold Foil Experiment submitted by:
Robert B. Gregory Department of Chemistry, Indiana University Purdue University Fort Wayne, Fort Wayne, IN 46805-1499;
[email protected] checked by:
Ed Vitz Department of Chemistry, Kutztown University, Kutztown, PA 19530
Demonstrations and analogies are used to great advantage in introducing new concepts in general chemistry. But where no examples exist that can be performed in the classroom, we turn to analogies as a way to give the students a useful mental image of the concept or principle. One such subject, where actual demonstration is, to say the least, impractical, is the Rutherford–Marsden–Geiger gold foil experiment. In a general chemistry lecture, we are often faced with the fact that, to our students, chemistry appears as one monodirectional linear story—everything looks perfectly planned, and the experiments always work the way they were supposed to. But, of course, this is not really true. Science moves and grows in fits and starts with new insights pushing out the old. The Rutherford gold foil experiments represent a powerful example of how science develops and improves its interpretation of reality. It is a reminder that scientific knowledge is not a static entity. The data in the experiment violated the expectations of the scientists performing it and presented them with a puzzling discrepant event, much as we use discrepant events to teach our students to think about the world around them. This demonstration provides an opportunity, in a lecture setting, to discuss the elegant nature of both the experiment and the conclusions, and to show students how, in a just a short time, one unexpected result can forever change the way science looks at the world. Rutherford’s Experiment Only six years had passed since J. J. Thompson refined Kelvin’s “plum pudding” model of the atom when, in 1909, Ernest Rutherford and two of his laboratory scientists, Hans Geiger and Ernest Marsden, proposed and performed a set of experiments designed to confirm the Thompson model of the atom. They would fire positively charged alpha particles at thin foils of metal, starting with the titular gold. Rutherford’s expectation was that most of the particles would pass through the foil unimpeded, with only one or two percent of them being deflected one or two degrees from the beam. That was not exactly what happened. The experiment was laborious, and the scientists were lucky to have seen the effect at all (1). But once the data had been collected, they were left with a very perplexing problem. The number and range of deflected alpha particles was much larger in the data than theory had suggested. In fact, 626
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about four percent of the particles were deflected out of the beam. On repeating the experiments, Marsden decided to look outside the small angle window and found that, additionally, about 1 in 8,000 particles were deflected at angles greater than ninety degrees—they were actually reflected by the foil (2). It took Rutherford nearly two years to explain those results. But in a tour de force (pun intended), he formulated what we now refer to—and teach to our first-year students—as the nuclear atom. The Demonstration Obviously, a direct, in-class demonstration of the Rutherford–Geiger–Marsden experiments is beyond the limits of practicality and safety. Few of us have the necessary equipment (or legal clearances) needed to handle radium or other alpha sources. And collecting enough data to actually see the distribution would take longer than any class period would allow. Thus, we must turn to a model of the experiment to demonstrate the effect. There are several Internet simulations of the various models of the atom, and several well-constructed simulations of the metal foil experiments that Rutherford’s laboratory performed (3, 4). This Journal has published several class participation exercises and demonstrations for illustrating the experiment (5–8). But in a lecture in which a simple presentation of the ideas surrounding the experiment is the goal, an effective demonstration of the dilemma Rutherford faced provides a good introduction to the striking experimental insight and the unique opportunity for new theoretical development that these experiments represented. This exercise first demonstrates an analogy to what Rutherford expected Geiger and Marsden to find and then demonstrates another analogy to what they actually found. The demo consists of two picture frames and a laser pointer. The instructor places two ring stands on the lecture table. One holds the laser pointer fixed in a three-fingered clamp, while the other holds a picture frame. The lecturer prefaces the demo with a description of the plum pudding model and what was to be an elegant confirmation of the structure of the atom: they expected very little scattering. When the laser pointer is turned on, the beam shines through the frame and strikes a screen or an unpopulated side wall, showing a single beam of light, unimpeded by any intervening object (Figures 1 and 2).
Vol. 84 No. 4 April 2007
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www.JCE.DivCHED.org
In the Classroom
The lecturer continues, announcing that the only problem is that it is not the way the experiment turned out. The picture frame is replaced with a second frame. When the laser hits that frame, the wall is again illuminated with a similar strong beam of light, but it is surrounded by scattered smaller points of light that indicate that some of the light has been deflected out of the beam as it struck the target (Figure 3). The rest of the lecture topic follows naturally from that point. Construction
Frame Two 5-in. × 7-in. picture frames purchased from a department store were used. The glass and backing plates were removed from both frames. The first frame is left empty. This frame is the analogy to what Rutherford expected to find. The second frame holds a plastic sheet with a surface that is rough on the scale of the laser beam wavelength. This frame is the “scattering frame” and represents what was actually found in the gold foil experiment. In the final form of the demonstration, an ink-jet printer film was used as the scattering medium. The plastic sheet was fitted in the picture frame or to a matte board, so that it is stationary and flat, as opposed to simply manually holding the film. This arrangement served two purposes. First, if the plastic sheet is handheld, the projected image moves and blurs considerably, making the effect very hard to see. Second, hand holding the film makes it difficult to control the retro-reflection, which is still a beam of laser light. Laser pointers are typically designed as Class II devices (
