Chemistry: Animation, Video, and Laboratory Activity

By 1909, Ernest Rutherford had established that alpha particles are positively charged particles. They are emitted at high kinetic energies by some radioactive atoms, that is, atoms that disintegrate spontaneously. In 1910, Rutherford’s research group carried out a series of experiments that had an enormous impact on the scientific world. They bombarded a very thin piece of gold foil with alpha-particles from a radioactive source. A fluorescent zinc sulfide screen was placed behind the foil to indicate the scattering of the alpha-particles by the gold foil (Figure 5-4). Scintillations (flashes) on the screen, caused by the individual alpha-particles, were counted to determine the relative numbers of alpha-particles deflected at various angles. Alpha particles were known to be extremely dense, much denser than gold.
If the Thomson model of the atom were correct, any alpha-particles passing through the foil would have been deflected by very small angles. Quite unexpectedly, nearly all of the alpha-particles passed through the foil with little or no deflection. A few, however, were deflected through large angles, and a very few -particles even returned from the gold foil in the direction from which they had come! Rutherford was astounded. In his own words, It was quite the most incredible event that has ever happened to me in my life. It was almost as if you fired a 15-inch shell into a piece of tissue paper and it came back and hit you.
Rutherford’s mathematical analysis of his results showed that the scattering of positively charged alpha-particles was caused by repulsion from very dense regions of positive charge in the gold foil. He concluded that the mass of one of these regions is nearly equal to that of a gold atom, but that the diameter is no more than 1/10,000 that of an atom. Many experiments with foils of different metals yielded similar results. Realizing that these observations were inconsistent with previous theories about atomic structure, Rutherford discarded the old theory and proposed a better one. He suggested that each atom contains a tiny, positively charged, massive center that he called an atomic nucleus. Most alpha-particles pass through metal foils undeflected because atoms are primarily empty space populated only by the very light electrons. The few particles that are deflected are the ones thatcome close to the heavy, highly charged metal nuclei.
Rutherford was able to determine the magnitudes of the positive charges on the atomic nuclei. The picture of atomic structure that he developed is called the Rutherford model of the atom.