ATOMIC STRUCTURE

Film : Rutherford Atom

Length(min.):40, Color: No, Sound: Yes, a PSSC Film R. Hulsizer, Univ. of Illinois

Qualitative experiments and appropriate models are used to indicate the ideas leading to Rutherford's nuclear concept of the atom. Its historical verification through the alpha particle scattering experiments of Geiger and Marsden is discussed.

The film should be used with Chapter 32 of the PSSC text.

Observing alpha particles in a small cloud chamber, Professor Hulsizer shows that a thin gold foil placed in their path apparently does little more than shorten the observed tracks. He describes an experiment in which Geiger examined the character of the small angle deflections of alpha particles in a narrow beam passing through a foil. Geiger's data showed that most alpha particles are not deflected out of the beam by more than one degree, and none beyond five degrees.

Using a similar scattering set-up but with a wider beam of alpha particles, Professor Hulsizer demonstrates an experiment done by Marsden which establishes that, although by far most of the particles are undeflected, a few are indeed deflected through very wide angles by the gold foil.

A mechanical scattering model is used to suggest the kind of atom that might explain these wide-angle deflections. Rutherford's concept that the atomic mass is concentrated in a small scattering center is discussed. Scattering by a Coulomb force is illustrated, using a Van de Graaff generator as the scattering center. With a three-dimensional model of the nuclear atom, the major aspects of a theoretical calculation for the distribution of scattered alpha particles are clearly pointed out. A comparison is made of the results of Rutherford's calculations with the experimental results of Geiger and Marsden in 1913, confirming the nuclear model of the atom.

Rutherford's estimate of the size of a gold nucleus, less than 10 exp -11cm, is discussed and compared to atomic dimensions.