In the Classroom edited by
tested demonstrations
Ed Vitz Kutztown University Kutztown, PA 19530
VSEPR Theory Demo Submitted by:
Janice Parker Department of Chemistry Demo Lab, University of Wisconsin–Madison, Madison, WI 53706
Checked by:
Cole McWherter and George Gilbert Department of Chemistry, Denison University, Granville, OH 43023
An easy, inexpensive method to demonstrate VSEPR theory involves the use of only a ring stand, clamp, 6 cow magnets,1 and a ball bearing 1-in. in diameter. For convenience, use an indelible marker to mark the similar pole of each magnet (either N or S). Fasten the clamp to the ring stand so that the grasping surfaces are vertical and as far from the stand’s upright rod as possible. Clamp one cow magnet vertically on the ring stand (as with the S pole oriented downward). Place the steel ball on the S pole (magnetic attraction will keep it in place). When the N pole of the second magnet is placed on the steel ball, it will orient itself in a vertical line with the first—as a linear molecule. When the N pole of the third magnet is placed on the steel ball a “molecule” with a trigonal orientation is demonstrated. Attaching the N pole of the fourth magnet, with care, produces a tetrahedral orientation; carefully attach-
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ing the fifth magnet produces a trigonal bipyramid; and the sixth magnet produces an octahedron. The spatial orientations produced this way are somewhat distorted downwards because of the gravitational pull on the magnets, an obvious disadvantage. But a significant and pertinent advantage of this method is the production of a series of models using invisible magnetic repulsion forces to show the orientations occurring in molecules where similarly invisible electron repulsion forces are at work. Note 1. Cow magnets are powerful cylindrical magnets used in a cow’s first stomach to attract ingested metal objects, such as nails, and keep them from traveling further through the digestive system. They can be obtained from Flinn Scientific (cow magnet, AP1944, $10.64 each). Similar bar magnets could be substituted.
Journal of Chemical Education • Vol. 74 No. 7 July 1997
