Before Ernest Rutherford’s landmark experiment with a few pieces of metal foil and alpha particles, the structure of the atom was thought to correspond with the plum pudding model. In summary, the plum pudding modelwas hypothesized by J.J. Thomson (the discoverer of the electron) who described an atom as being a large positively charged body that contained small, free-floating, negatively charged particles called electrons. The plum pudding model also states that the negative charge of the electrons is equivalent to the positive charge of the rest of the atom. The two charges cancel each other causing and cause the electrical charge of the atom to be zero (or neutral). The faulty aspect of this model is that it was construed before the nucleus of an atom (and its composition) was discovered; which is where Rutherford’s research comes in.
*Note – For more information on the research of J.J. Thomson and the plum pudding model, click here .
A large portion of Rutherford’s research has always included the use and study of alpha particles ever since he classified them in 1898 (Rutherford’s association with alpha particles was discussed here ). Starting sometime around 1909, Rutherford began to notice that alpha particles would not always behave in accordance to the plum pudding model of an atom when fired at a piece of gold foil. These observations stimulated further research that was eventually published in 1911 and has been known ever since as Rutherford’s Gold Foil Experiment.
Throughout the course of his experiment, Rutherford had his two associates (Hans Geiger and Ernest Marsden) aim a beam of alpha particles at a piece of gold foil that was approximately 8.6 x 10^(-6) centimeters thick. To be more accurate Rutherford actually included a wide variety of different foils (such as: aluminum, iron, and lead), but his use of gold foil is most commonly spoke of. In accordance to the J.J. Thomson model of an atom, the alpha particles should have passed directly through the gold foil for all instances. Therefore to confirm this activity, a zinc sulfide screen was placed behind the foil as a backdrop for the alpha particles to appear upon. Directly above this screen was a microscope that allowed one of the two experimenters (only Geiger and Marsden actually performed the experiment, Rutherford just explained the results) to observe any contact made between the alpha particles and the screen. In order for the light of the alpha particles to be observed, the experiment was performed in complete darkness. Also, to further enhance the accurateness of the observations the experimenter that was charged with looking through the microscope sat in the dark of the lab room for at least one hour before performing the experiment. This was done in order to allow the experimenter’s eyes to reach maximum visual acuity.
After the experiment had been set up in accordance to the speculations described above, Geiger and Marsden would fire the beam of alpha particles through the piece of foil and observe the location at which the particles landed on the screen. As explained above, each particle should have gone directly through the foil if the plum pudding model was correct (meaning that an atom was a vast amount of empty space and could easily be passed through by any particle). For the most part, the alpha particles corresponded with this hypothesis and passed straight through the gold foil. There were, however, a small hand full of particles that deflected slightly from the straight path by about one or two degrees. But the biggest discovery was made when 1 in 20,000 particles would deflect approximately 90 degrees or more from the parent beam. In fact, an occasional particle even fired right back at the experimenter. Perhaps Rutherford described the awe inspiring nature of the discovery best when he said: It was as if you fired a 15-inch shell at a sheet of tissue paper and it came back to hit you. To help illustrate what Geiger and Marsden observed, a small demonstration is presented below.