The Gold Foil Experiment

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 model was 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.

The results of this experiment gave Rutherford the means to arrive at two conclusions: one, an atom was much more than just empty space and scattered electrons (J.J. Thomson model argued), and two, an atom must have a positively charged center that contains most of its mass (which Rutherford termed as the nucleus). Since alpha particles are relatively heavy, positively charged particles, the fact that the occasional particle would be deflected by either a small or large margin proved that a portion of an atom was both positively charged (particles of identical charges repel one another while particles of opposite charges attract one another) and relatively heavy by atomic standards. Since only a small number of alpha particles veered slightly when passed through the foil and since even fewer bounced back at the experiment, Rutherford reasoned that this positively charged center was relatively small in reference to the total size of the atom. Therefore J.J. Thompson was slightly correct in his assumption that atoms are primarily composed of empty space.

Given below is an illustration that compares the plum pudding model of an atom to what Rutherford observed in his experiment. The top structure shows how the alpha particles would have passed through the gold foil atoms if the plum pudding model was correct in its assumptions. The bottom structure shows what Rutherford and his collogues observed and is the true depiction of an atom's structure.

With the disproof of the plum pudding model and with Rutherford's discovery of an atom's nucleus, it was now possible for Neils Bohr to construct his model of an atom's structure. Bohr conversed with Rutherford on several occasions and was able to use the knowledge he gained from the encounters to create what is now called the quantum mechanical model of an atom. To explain briefly, Bohr described the hydrogen atom as a nucleus with an electron circling around it, much as a planet orbits the sun. The problem with Bohr's model is that it only works soundly for the hydrogen atom (this and other facts about Bohr's research and the quantum mechanical model can be found here). Even though Bohr's model was not entirely correct in its application, the point to grab here is that Rutherford's experiment with gold foil and his discovery of the nucleus had a huge contribution to the development of today's atomic theory(s). Yes another scientist probably would have made the same discovery given the opportunity of a few years, but Rutherford had the intelligence and the incite to put the ideas together when he did. Essentially, putting ideas together is really what science is all about.