Perhaps surprisingly, the Rutherford gold foil experiment was not actually performed by Nobel laureate chemist Ernest Rutherford, though it was carried out under his supervision, by his junior colleagues at the University of Manchester, Hans Geiger and Ernest Marsden. The experiment directed a beam of radioactive alpha rays onto gold foil, and finally disproved the atomic plum pudding model, leading to the rise of Rutherford’s competing orbital model.
When the gold foil experiment was carried out in 1909, atomic theory was still a new and controversial field. Earlier in the decade, J.J. Thomson, the discoverer of the electron, had theorized that atoms were made up of relatively coherent blocs of negatively charged electrons, surrounded by a mass or “pudding” of positively charged protons, whose positive charge would contain the structures of electrons. Thomson’s model quickly picked up several other nicknames, as well, such as the “chocolate chip cookie model,” although to Thomson himself it was important that the electrons could rotate freely so long as they remained within the protective structure of protons.
THE GOLD FOIL EXPERIMENT
Rutherford and his colleagues were dissatisfied with the theory, however. In the famous 1909 gold foil experiment, they surrounded a thin sheet of gold foil with a second sheet made up of zinc sulfide, and then bombarded the apparatus with alpha rays. Alpha rays, or alpha particles, are the largest form of radiation, a chunk of subatomic material consisting of two protons and two neutrons which is fired out of a decaying atomic nucleus. (For more on the importance of radioactive decay, see this page.) Zinc sulfide was used to detect the presence of the alpha rays, because it lights up when exposed to this form of radiation.
Under Thomson’s plum pudding theory, the alpha rays should have more or less passed directly through the gold, occasionally being deflected a few degrees off course as they passed by one of the various electron structures dispersed throughout the atom. Instead, however, something quite different happened: a small percentage of the beam of alpha rays was deflected, but instead of being slightly askew, it flew in wild directions, sometimes more than 90 degrees off course.
THE RISE OF THE RUTHERFORD MODEL
As Rutherford noted in the paper which presented the results of the gold foil experiment to the scientific community, the plum pudding model could not explain the results of the gold foil experiment: in that model, there simply was no subatomic structure large enough to lead to observable collisions with alpha particles. The best alternative explanation, he argued, was that most of the mass of the atom was actually clustered in a single, relatively heavy nucleus. Most of the alpha rays would miss the nucleus; however, when they hit it, they would be deflected far off course, as Geiger and Marsden had observed in the experiment.
Essentially, Rutherford’s new model – which is still the basis for modern chemistry – reversed Thomson’s model. Instead of groups of electrons floating in a pudding of protons, the protons were densely packed together in the core of the atom, and the electrons were the ones sitting outside. In his model, the electrons actually orbited the nucleus following regular, patterned paths, a model established by way of analogy to the planets orbiting our Sun. Chemists today believe that electrons actually follow much more diverse, random paths, although the shells in which they are contained still function in a similar manner to the orbits proposed by Rutherford.