Exploration:

Rutherford’s Prediction

The purpose of this exploration is to investigate the Thomson model of the atom in order to understand what Rutherford expected as he set out to conduct his famous scattering experiment, since his prediction was based on the Thomson atomic theory. To assist you as you work through this exercise, click on the toolbox to the rightto open the “Thomson Model” applet. If you need help understanding the apparatus and variables, see the Help menu for assistance before starting this exploration.

While Rutherford was studying radioactive decay at McGill University in Montreal, J.J. Thomson was proposing a new atomic model to replace the Dalton model. In a paper published in 1904, Thomson wrote “… atoms of the elements consist of a number of negatively electrified corpuscles enclosed in a sphere of uniform positive electrification …” (On the Structure of the Atom, J.J. Thomson, Philosophical Magazine, Series 6, vol. 7, Number 39, p. 237, March 1904.)

Your Turn

  1. According to Thomson, where is the positive charge of the atom? How does this relate to the analogy of a “raisin bun”?
  1. Theories are initially created to describe and explain some known evidence. What is the main evidence that inspired Thomson to create his atomic theory?
  1. Describing and explaining are the first and easiest criteria for a theory to meet. What is the most difficult test of a theory? If necessary, see theCRYSTAL-Alberta site.

Using the applet, reduce the energy of the alpha particle to 1 MeV and press the Auto-fire button. Let the applet run until you have collected a few tracks above and below the central line.

  1. Sketch the results and describe them in a sentence or two.
  1. Click the Protractor button and determine the largest angle of deflection (scattering angle).

Leaving the tracks on the screen, adjust the energy to 10 MeV. Let this run until you have collected a few tracks above and below the central line.

  1. Describe the results compared with 1 MeV alpha particles. Explain briefly, including appropriate calculations.
  1. Click the Protractor button and determine the largest scattering angle (if there are too many lines, click Clear Tracks, set the impact parameter above or below the central line, and click Fire to obtain a single track).

Press Clear Tracks. Try several different energies of alpha particles at several different impact parameters.

  1. At what impact parameters is the scattering angle the greatest? Suggest an explanation.
  1. Does the Thomson model ever stop the alpha particle? Why or why not?
  1. The purpose of scientific work can be described as a create-test-use (CTU) process (see theCRYSTAL Alberta site). Identify and describe the purpose of Rutherford’s work.

“The theory of Sir J. J. Thomson is based on the assumption that the scattering due to a single atomic encounter is small, and the particular structure assumed for the atom does not admit of a very large deflexion … .” (The Scattering of  and  Particles by Matter and the Structure of the Atom, E. Rutherford, Philosophical Magazine, Series 6, vol. 21, p. 670, May 1911.)

  1. Identify the hypothesis and prediction that Rutherford was using.

Rutherford also knew about and considered the Nagaoka model:

“It is of interest to note that Nagaoka has mathematically considered the properties of the Saturnian atom which he supposed to consist of a central attracting mass surrounded by rings of rotating electrons. He showed that such a system was stable if the attracting force was large. From the point of view considered in his paper, the chance of large deflexion would practically be unaltered, whether the atom is considered to be disk or a sphere.” (The Scattering of  and  Particles by Matter and the Structure of the Atom, E. Rutherford, Philosophical Magazine, Series 6, vol. 21, p. 688, May 1911.)

1