The Gold Foil Experiment

Ernest Rutherford’s Gold Foil Experiment was a major stepping stone one the way to discovering what the atom was really made up of. From the beginning of his research with alpha particles to his discovery of the atomic nucleus, Rutherford made many contributions to the microscopic world of the atom. The Rutherford Experiment, otherwise known as the Gold Foil Experiment, was the crown of his achievements, and it was during this experiment that he discovered the atomic nucleus. (Aydin &Hanuscin, p.59) He made this happen by putting his past research on alpha particles to the test, such as the knowledge that an alpha particle should be less massive than the atom. Therefore the alpha particles should pass directly through the atom, unfazed. When the particles started to reflect off of the atoms in obtuse angles, Rutherford began to question the plum pudding model of the atom. It did not take him long to scientifically prove the model wrong, it only took about 2 years after the initial experiment to get enough proof to disprove the Plum Pudding Model and publish his own atomic model, The Rutherford Model. This article will be focusing on Rutherford’s Gold Foil experiment and how the experiment’s results changed how scientists would look at the structure of the atom for many years to come. (Nagendrappa, p.1013,1014)

The Gold Foil Experiment
In the years before Ernest Rutherford and his associates, Hans Geiger and Ernest Marsden, did the Gold Foil experiment, scientists explained the structure of the atom using the Plum Pudding Model. The Plum Pudding model stated that the atom was solid and only had protons and electrons mixed together in a single piece of matter. In 1906, Rutherford, Geiger, and Marsdon decided to test the Plum Pudding model by firing alpha particles at a piece of gold foil. Their results would revolutionize the model of atomic structure to one that is similar than the model used today. By examining the Gold Foil Experiment one can see that it paved the way to newer models of the atom by the discovery of the atomic nucleus. (Nagendrappa, p.1011,1012) The History of the Gold Foil Experiment

Rutherford’s study of Alpha particles began in the late nineteenth century, when he was residing in Manchester. The concepts of Alpha particles were Ernest Rutherford’s major field of study since the ending years of the nineteenth century. His first major discovery dealing with these alpha particles was that a very small portion of alpha particles were slightly deflected off of the atoms they were fired at. (Nagendrappa, p.1011,1012) With the help of Geiger, Rutherford was able to conduct a series of experiments to test this scattering of alpha particles as they collided with a thin mass. This phenomenon had left Rutherford questioning the Plum Pudding Model of the atom, because according to that, the atom should not have anything massive enough to deflect the alpha particle. With his newfound knowledge, Rutherford set out to prove the plum pudding model wrong and find the truth of what is inside of the atom. With this in mind, he and his associates set up what is known as the Gold Foil Experiment to test their hypothesis. However they did not know that the results of their experiment would forever change the way the world looked at the structure of the atom. (Nagendrappa, p.1011-1014) The Experiment

The experiment itself had a simple setup, only consisting of an alpha particle laser, a screen that is coated by zinc sulfate, and a super thin plate of gold foil. The zinc sulfate reacted with the alpha particles creating sparkles on the screen when the two collided. This was extremely useful to finding where the particles hit, showing if their original path that they had been traveling had been altered in any way. These sparkles would be hard to see in a lit room so the experiment would have to be conducted in a dark room. The alpha particles used for the experiment would be projected from an apparatus consisting of Polonium in a lead box. The foil would be suspended off of the ground and almost completely surrounded by the zinc sulfide coated screen. The screen would also be at the focus of a microscope that would be used as the detector; this microscope would be pivoted around the foil in order to observe particles from any given angle. (Aydin & Hanuscin, p.59) When they had started the experiment, everything was going as predicted.

Most of the particles were either going straight through the gold foil or getting slightly deflected. This all changed when an alpha particle was reflected almost straight back at the apparatus. Other particles started to reflect at approximately ninety degrees off of the foil, in fact after going through more trials, Rutherford concluded that one in twenty-thousand alpha particles were being deflected on an average angle of ninety degrees.(Colvin, p.46) This fact astounded Rutherford, Geiger, and Marsdon, because according to the current model of the atom there should not have been anything with enough concentrated mass inside the atom to stop the alpha particle from passing straight through the atom, without a collision. Rutherford said that, “…It was almost as incredible as if you had fired a 15-inch shell at a piece of tissue paper and it came back and hit you.” (Aydin & Hanuscin, p.59) This brought up a series of questions, with one of them being a forerunner to further trials: just what really is in the middle of an atom? With this question fueling them, the three scientists continued to conduct a series of the same experiment to gather enough data to scientifically prove the Plum Pudding Model wrong. (Colvin, p.46) Results of the gold foil experiment

There were many results of The Gold Foil experiment, these results allowed Rutherford to come to two conclusions: one, an atom was much more than just empty space and scattered electrons, and two, an atom must have a positively charged center that contains most of its mass. The conclusion that an atom was much more than just empty space, came from the simple fact that a small portion of the alpha particles got deflected at angles that averaged out to be approximately ninety degrees. Where these alpha particles were deflected, there would have to be something massive and have a charge that would repel the alpha particle in the center of the atom. (Colvin, p.46)

The explanation for the conclusion that an atom must have a positively charged center is that it would take something of the same charge and of higher mass than the alpha particle to deflect it in an obtuse angle. Rutherford called this positively or negatively charged center the Nucleus; he never did specify what charge he thought the nucleus had. (Colvin, p.46) He published the term, nucleus, along with his own data and model of the atom in 1911. He named his model after himself, calling it the Rutherford model and it was the closest interpretation of the structure of the atom at this point in time. (Nagendrappa, p.1012-1014) Analysis

After realizing that the alpha particles had deflected at either an acute angle or an obtuse angle, Rutherford came to the conclusion that there was a central force in the atom that repelled the positively charged alpha particles. He also came to the conclusion that the said central force had to be a very small portion of the atom, because the majority of the alpha particles passed directly through the atom. The plum pudding model stated that the atom was a uniform mass, and where Rutherford discovered the nucleus he proved the Plum Pudding model wrong. Most of the atoms mass resided in the nucleus and the rest of the atom was empty space, save the electrons floating around in it. (Aydin & Hanuscin, p.59) Conclusion

Before 1911, any if one would have wondered about the structure of the atom, they would have to study the Plum Pudding model. However after 1911 and the discovery of the atomic nucleus, the views of the atomic structure drastically changed. This drastic change in the view of the atomic structure has three early twentieth century scientists to thank, Rutherford, Geiger, and Marsdon. It was their Gold Foil Experiment that discovered the atomic nucleus and brought a new era of atomic structure to the world of science. As one examines the Gold Foil experiment one can tell that its results only accounted for change in the view of the atomic structure. (Nagendrappa, p.1011,1012)

References

Colvin J. Ernest Rutherford and the gold foil experiment. New Zealand Science Teacher, (130),46. Nagendrappa, G. (2011). Ernest Rutherford. Resonance: Journal Of Science Education, 16(11), 1007-1018. doi:10.1007/s12045-011-0113-1 Aydin, S., & Hanuscin, D. L. (2011). Secret in the Margins: Rutherford’s Gold Foil Experiment. Science Teacher, 78(2), 56-60.