When J. J. Thomson discovered the electron in 1897, people understood that the atom was not the smallest unit of matter.  Moreover, because electrons are negatively charged but atoms are neutral, atoms must also consist of a positively charged substance.  Recall that Thomson's model described the atom as electrons embedded throughout a positively charged substance. Thomson's model is often called the raisin-bun model or the plum pudding model.

Thomson's model had a relatively short lifespan.  As experimentation progressed and evidence gathered, people understood that within the atom there must exist an extremely tiny, but very massive, positively charged core, giving rise to the Planetary Model of the Atom.  Evidence supporting this model was collected by Ernest Rutherford and his assistants, Hans Geiger and Ernest Marsden, who observed the scattering of positively charged particles as they encountered a thin layer of gold atoms.

In their experiment, positively charged helium ions (called alpha particles) from a small sample of radioactive radium were used to bombard gold atoms.  When the charged alpha particles encountered gold atoms, they were scattered at various angles.  The scattered alpha particles could be observed when they encountered a zinc sulfide screen attached to a microscope.


Read
Read "Rutherford's Scattering Experiment" on pages 767 and 768 of the textbook.


Watch This
Open the Rutherford Scattering simulation to see how a large nucleus scatters smaller, charged alpha particles. (Firefox may be required.)

In the simulation you will notice that alpha particles are composed of two red protons and two grey neutrons without any electrons, producing the characteristic +2 ion charge.  

Also notice the small but very distant electron that orbits the large nucleus.  You will see it pass along its circular path in the corners of the viewing area, giving a sense of how small the nucleus and electron are relative to the majority of empty space in the planetary model of the atom.

Self-Check

Answer the following self-check question then click the "Check your work" bar to assess your responses.

SC 1. Adjust the number of protons on the atom using the simulation slider. Set it to 20 protons. Select "Show Traces" to see the path of each alpha particle. Use the term many, few, or rare to complete the following three statements:

  1. _______ of the alpha particles pass by with little or no scattering, indicating the atom was mostly empty space.
  1. ______ of the alpha particles are scattered at large angles, indicating the presence of a small, dense nucleus.
  1. On occasion, _____ alpha particles are scattered straight back toward the source, indicating the presence of a very dense, positively charged nucleus. Presumably, a large electrostatic force of repulsion would be required to reverse the alpha particles' direction of motion.

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Self-Check


Contact your teacher if your answers vary significantly from the answers provided here.

SC 1.

  1. Many of the alpha particles pass by with little or no scattering, indicating the atom was mostly empty space.

  2. Few of the alpha particles are scattered at large angles, indicating the presence of a small, dense nucleus.

  3. On occasion, rare alpha particles are scattered straight back toward the source, indicating the presence of a very dense, positively charged nucleus. Presumably, a large electrostatic force of repulsion would be required to reverse the alpha particles' direction of motion.