The Rutherford Atomic Model: Hidden Obstacles

Advanced Version

Background:

In 1911 Ernest Rutherford and his co-workers conducted an experiment in which they directed a narrow beam of alpha particles at a thin piece of metal foil. Expecting the particles to shoot straight through to the other side, they were surprised to find that a percentage of the alpha particles were actually reflected back toward the source or “scattered” at large angles due to their encounters with the metal atoms in the foil target. Alpha particles are small, positively charged, high-energy particles that travel at about 1/10 the speed of light. It became clear that only strong forces within the atom were deflecting the alpha particles. The atoms making up the target, however, presented Rutherford with a kind of black box; the structure of the atom was not known at the time.

Key Concepts:

  • Most of the mass of the atom is concentrated in a very small, dense central area, later called the nucleus, which is about 1/100,000 the diameter of the atom.
  • The rest of the atom is apparently “empty space”
  • The central, dense core of the atom is positively charged, with the nuclear charge equal to about one-half the atomic mass.
  • Electrons occupy the bulk of the empty space in an atom and orbit the nucleus at a distance.

The purpose of this activity is to discover by indirect means the size and shape of an unknown object, which is hidden underneath the middle of a large board.By tracing the path the marble takes after striking the unknown target from various sides, it should be possible to estimate the general size and shape of the unknown target.

Materials:

  1. A ~60x45cm cardboard platform with “unknown” object attached
  2. One marble (or more in case marble rolls away)
  3. One white standard 8.5in x 11in sheet of paper
  4. One Dark marker
  5. Two rulers
  6. Scotch Tape

Pre-Lab Questions:

  1. This activity is a simulation of Rutherford’s scattering experiment. Read the entire procedure and compare the components used in this simulation to Rutherford’s original. Discuss what each component in our simulation corresponds to in the original experiment.
  1. The key skills in this activity, as in Rutherford’s experiment, is the ability to make careful observations to draw reasonable hypotheses, and the ability to determine the shape of an unknown object through indirect means.In the box below, the marble strikes various sides of a possible target.Sketch the paths the marble is likely to take, according to the direction it is rolled in, and the rebound it will have when it makes contact with the target.
  1. Discuss what information can be inferred if the marble rolls straight through without striking the unknown target.

Procedure: Rectangle

  1. Stand in front of one side of the cardboard platform.
  2. Roll marble with moderate force and speed under cardboard platform. Marble must be aimed in a straight path every time.
  3. Observe where the marble comes out from and with the dark marker trace the approximate path of the marble on the sheet of paper that is taped on top of board.
  4. Repeat rolling marble (and tracing paths) at least 20 times on same side of board.
  5. After marble has been rolled at least 20 times on one side of the board, move on to another side of the board and repeat process until marble has been rolled under cardboard from all sides.
  6. After sketching (on the sheet taped on the upper side of the cardboard) the apparent paths of the marble from all sides, the general size and shape of the unknown target should emerge.
  7. Form a working hypothesis concerning the structure of the unknown target. Based on this hypothesis, on the sheet of paper you are using to trace the path of the marble, outline the shape you think the object lying directly underneath the board has.
  8. Draw the general size and shape of the target to approximate scale in the square below:
  1. Measure length and width of the outline you drew on the sheet of paper.
  2. Turn cardboard over and find out what the object is, then take actual measurements of object.
  3. Check actual measurements with teacher.

Post Lab Questions:

  1. Compare your object’s sketch to the actual shape of the object. Were you close?
  1. Compare your estimated measurements to the object’s actual measurements. Were you close?
  2. The speed of the marble rolls was an uncontrolled variable in this activity. How would the outcome of the scattering test have been different if the marble speed had been faster or slower?
  1. Compare the overall size of the target with the size of the marble used to probe its structure.How would the outcome of the scattering test have been different if different size marbles had been used? Explain.

Procedure: Unknown Shape

  1. This time, a different object is placed under the cardboard platform.
  2. Center sheet of paper on the upper side of the cardboard platform and tape down.
  3. Stand in front of one side of the cardboard platform.
  4. Roll marble with moderate force and speed under cardboard platform. Marble must be aimed in a straight path.
  5. Observe where the marble comes out from and with the dark marker, trace the approximate path of the marble on the paper.
  6. Repeat rolling marble (and tracing paths) at least 20 times on same side of cardboard.
  7. After marble has been rolled at least 20 times on one side of the cardboard, move on to another side of the cardboard and repeat process until marble has been rolled under cardboard from all sides.
  8. After sketching the apparent paths of the marble from all sides, the general size and shape of the unknown target should emerge.
  9. Form a working hypothesis concerning the structure of the unknown target. Based on this hypothesis, on the sheet of paper you are using to trace the path of the marble, outline the shape you think the object lying directly underneath the board has.
  10. Draw the general size and shape of the target to approximate scale in the square below:
  1. Measure the length and width of the shape you drew on the white sheet of paper.
  2. Turn cardboard over and find out what the object is, then take actual measurements of object.
  3. Check actual measurements with teacher.

Questions:

  1. Compare your object’s sketch to the actual shape of the object. Were you close?
  1. Compare your estimated measurements to the object’s actual measurements. Were you close?