The Rutherford gold-foil experiment, although actually developed and carried out in 1909 by members of Rutherford’s research team, drove what was perhaps the most important paradigm shift concerning the structure of matter known to history.
Early concepts of the composition of matter focused on the Greek notion of an atom (atamos Gr.), which means an entity which cannot be divided. However, in 1897 J.J. Thomson’s discovery of the electron showed that atoms could indeed by divided into positively and negatively charged pieces. In an amazing series of experiments that won him the Nobel Prize in 1906, Thomson found that the electron was negatively charged and far lighter than the atoms from which they had been displaced. Because chemical elements are uncharged, the negative electronic charge shows there is another part of an atom which must be positively charged. This led in 1904 to Thomson’s plum pudding model of the atom, in which electrons are suspended in a uniform positively charged ‘pudding’ which supplies most of the mass of the atom.
The plum pudding model did not last long. In the latter years of the first decade of the 20th century, Hans Geiger and Ernest Marsden were working in Ernest Rutherford’s laboratory at the University of Manchester. Their status was roughly equivalent to post-doctoral researcher and undergraduate student. (It is interesting to note that Thomson was Rutherford’s Ph.D. thesis advisor.)
In 1908, Geiger had developed an apparatus, suggested by Rutherford, to investigate the scattering of energetic alpha particles from various types of foil. The source of alpha particles was a tube of ‘radium emanation’, which is a mix of radon isotopes which are given off by radium as it undergoes radioactive decay. A narrow beam of these alpha particles was directed against a gold foil whose thickness was several hundred atoms. The path of the alpha particles was changed by scattering from the gold atoms, and the path deflection from the original path was measured by observing the light emitted when the scattered alpha particles hit a zinc sulfide screen.
Initial investigations did show the expected scattering through small deflection angles, and the study was temporarily shelved. In 1909, Geiger approached Rutherford with the idea that Marsden had reached the point where he needed a research project of his own. Rutherford suggested that he study the large angle scattering of alpha particles from gold foil using Geiger’s apparatus.
Geiger and Marsden carried out this study, and discovered that the deflection angles were much greater than predicted by the plum pudding model. About one in 20000 alpha particles were deflected through an angle greater than 90 degrees – in fact, some of the alpha particles were bounced directly back at the source. Rutherford’s comment on this observation: “It was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you. On consideration, I realized that this scattering backward must be the result of a single collision, and when I made calculations I saw that it was impossible to get anything of that order of magnitude unless you took a system in which the greater part of the mass of the atom was concentrated in a minute nucleus. It was then that I had the idea of an atom with a minute massive centre, carrying a charge.”
Rutherford and his coworkers followed up this remarkable discovery with additional experiments and calculations, leading in 1911 to his historic Philosophical Magazine paper titled “The Scattering of α and β Particles by Matter and the Structure of the Atom”, which laid out the atomic model which would henceforth be called the Rutherford model.
Rutherford’s model of the atom proposed that all of the positive charge of the atom (and most of the mass as well) is concentrated within a tiny nucleus having a diameter about 10000 times smaller than the atom. The electromagnetic force between the electrons and the nucleus follows an inverse-square law, similar to the gravitational force that keeps the planets revolving around the Sun. In Rutherford’s model, the electrons orbit around the nucleus, held in place by this electromagnetic force.
This model is consistent with Rutherford’s remarkable scattering results, but still has serious problems. The context and background of the Rutherford model is classical physics. Any body which does not travel in a straight line undergoes acceleration, in this case to keep the electrons moving in an orbit around the nucleus. In classical electrodynamics, however, accelerating charges give off electromagnetic radiation, and lose energy as they do so. As the energy of the atom is radiated away, the electrons should spiral down toward the nucleus. There is no classical mechanism within the context of the Rutherford model which can explain the stability of matter. The explanation lies in quantum mechanics, a viewpoint which was not sufficiently developed in 1911 to enter into Rutherford’s atomic model.
Prior to the gold-foil experiment, the viewpoint of the atom as an essentially featureless blob was rarely questioned. Following the experiment, the idea of a complex substructure for the atom was universally accepted. Sometimes a key experiment changes everything – the gold-foil experiment is a superb example and a lesson for the future. Always allow for the possibility of the unexpected result.