- University of Virginia
- Physics Department
A Physical Science Activity
2003 Virginia SOLs
- conduct controlled experiments;
- record and organize experimental data;
- draw conclusions from collected data;
- describe the Rutherford Gold Foil experiment and its contributions to the
development of the Bohr model of the atom;
- explain the importance of using indirect measurement techniques to draw
conclusions about objects that cannot be seen;
- estimate the size of the unknown object based on known, comparable dimensions.
How can we measure the characteristics of objects that we cannot
see? How do we know the structure of an atom?
Often we can look at or touch an object to learn about it.
Sometimes, however, objects are too small or too large for us to
learn about them this way. When this happens, we need to use indirect
measurement techniques. Ernest Rutherford realized that atoms, which
are the building blocks of nature, are much too small to be measured
directly, and so he designed an experiment to measure their
characteristics indirectly. He used a thin piece of gold foil at
which he directed alpha particles, which were like very small
bullets. Though he could not see the atoms in the gold foil, he knew
that if he watched where the alpha particles went after hitting the
gold foil, he could draw conclusions about the gold atoms. Alpha
particles are very small, but they are heavy. They also travel
quickly, and they have a positive electrical charge. When the alpha
particles collide with a specially designed screen that Rutherford
placed around the gold foil experiment, the screen would light up at
the point of the collision.
Imagine a stream of water from a garden hose directed at a brick
wall. What would happen to the water as it impacted the wall?
Rutherford thought alpha particles against the gold foil would behave
much like water against the wall, but he was very surprised to find
most of the alpha particles went straight through the foil. Imagine
how you would feel if the water from the hose went straight through
the wall! This experiment led Rutherford to conclude that an atom is
actually mostly empty space with a small, dense, positively charged
nucleus in its center.
- Small wooden block (approximately 6 inches across) cut into a
triangular or circular shape or with irregular edges (must be
significantly smaller than the plywood)
- Large piece of plywood (or large, collapsed cardboard box like
those in which refrigerators are delivered; foamcore or
posterboard could also be substituted) which is much larger than
the block and can be placed over the block so that the block
cannot be seen (approximately 4'x 6') (Figure 1)
- Marble or steel ball bearing (a ping pong ball or golf ball
will also work)
- Supports for the corners of the plywood (spools work well)
- Blackboard or another surface to record data
- Hard, level floor
- On a level, hard floor, set up the plywood or large box as
shown (Figure 1). Center the wooden block to be used as the
"unknown object" under the plywood so it is not visible to the
students. (You may want to tape the small block in place so it
will not shift when struck with the marble during the experiment.)
(Marking off 5 cm increments on all edges of the board will aid in
students' identification of exit points.)
- Have the students form a circle around the plywood. (Figure 2)
- Draw a sketch of the plywood set-up on the board. Include on
it the dimensions of the plywood and the approximate positions of
the students around it. You may want to mark the positions on the
floor for the students to sit to ensure that they remain properly
spaced around the set-up. (Knowing their locations precisely will
also increase the accuracy of the experiment.)
- One student at a time will be the "Launcher" and will roll the
marble under the plywood on the floor. An area on one edge should
be designated as the "Launch Area" so that all trials start from
the same general location. The other students must be ready to
catch the marble when it comes out from under the board. The
marble should be launched fast enough that it will roll straight
through the set-up unless it collides with the center object.
- Record the path of the marble on a sketch of the set-up on the
blackboard. The path should include the exit point from under the
plywood and the position within the group of children where the
marble was caught. This helps visualize the angle of deflection
for the marble. Additionally, point out to the students that the
only way for the marble to be deflected is through collision with
the small object in the middle. Assume the marble's path is
straight line until it collides with the center object. Following
the collision, the path is again straight.
- Students can rotate through the launching position so each has
a chance to launch the marble. As students move to the launch
position, have other students move around the set-up to ensure
each position remains filled.
- After everyone has rotated through the launch position or
after sufficient data are collected, examine the recorded paths
followed by the marble. As a group, try to conclude the
approximate size and shape of the wooden block.
- If the marble collides with one of the supports, do not use
the trial. Re-launch the marble.
- If the marble fails to exit from under the plywood, do not
use the trial. Re-launch the marble.
- When recording the paths of the marble, indicate trials
where it collides with the unknown objects differently than those
where it does not collide (different colored chalk, for
Figure 1: Set-up for the activity. The unknown block is
placed at the center of the board which is supported at each
corner. In this case, rubber stoppers were used. Spools or
other small diameter objects will also work. The board has
been marked to help students identify the exit point of the
marble (shown in the foreground).
Figure 2: Arrangement of students around experimental
- Students use a computer
simulation of Rutherford's Gold Foil experiment (includes programs for
Bohr and Schrodinger).
- Students build a model of the Rutherford gold foil experiment.
- Students conduct a World Wide Web search for biographical information on
- Students compose a newspaper article about Ernest Rutherford and his contributions
to the atomic model.
Students with Special Needs
Students unable to participate in the rolling and catching of the marble could
serve as data collectors. Or, the participants could be limited to a fraction
of the class rather than the entire group while the remainder of the class observes
and conjectures on the shape and size of the unknown object.
Click here for further
information on laboratories with students with special needs.
- Students identify examples from everyday life where objects
are analyzed in a manner similar to the Rutherford Gold Foil
- Students place themselves in the position of Ernest Rutherford
following his gold foil experiment and write a short paper
explaining their feelings about the outcome of the experiment
along with their conclusions.