Follow the Bouncing Balls to an Understanding of Molecular Structure James P. Birk Arizona State University, Tempe, AZ 85287
Valence shell electron pair repulsion (VSEPR) theory provides a simple rationale for the shapes adopted by molecules ( I ) , so i t has become widely used for predicting and explaining covalent molecular structure. Because students have difficulty envisioning three-dimensional structures, various molecular models are invaluable in presenting this material. VSEPR Models
A variety of models have been proposed to illustrate the VSEPR approach to predicting molecular structure. Pairs of Styrofoam halls connected by rubber hands can he combined to provide the various shapes (2,3). Similarly, sets of balloons can be tied together to provide models of the various fundamental shapes corresponding to each set of two to six electron pairs staying as far apart as possible (4-6). A third variation can be found in the recent description of models created from Styrofoam balls connected by strips of Velcro (7). All of these approaches suffer from the need to manipulate the model extensivelv to assemble it, so it is not clear to the students that the shapes are indeed natural rather than contrived. T o demonstrate that manipulation is not required to achieve the desired shapes, it is-necessaryto distort the models and show that they will return to the natural shapes. A "hands-off" approach would be preferable since it would then be immediately apparent that the shapes arise naturallv. In a variation on the balloon model (5). six balloons are tied together and then are successi\.ely broken with a oin. While this ~rovidesthe hands-off a . o.~ r o a c hneeded, this demonstration still suffers from some deficiencies: It is not possible to reverse the demonstration, the model is not very permanent, and the demonstration starts with the less common high coordination numbers. ~~
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Figwe 1. The bounclng balls model is constructed from six rubber bells, which are connected to nylon strings. The strings are threaded through a wooden
tube and tied to weights.
Construction ol the Bouncing Balls Model
We have developed a model that has been used successfully and effectively in a number of courses at different levels (college and high school chemistry courses and workshops for in-service and pre-service high school chemistry teachers). The model was constructed of six hollow soft rubber balls (Fig. I),4 in. in diameter. These balls had small plastic plugs, which were removed to provide a means of fastening strings to the balls. Nylon string was tied to the largest washer that could be forced through the hole to attach the balls firmly to strings. The strings were first threaded through a wooden thermometer tube cut off a t the solid end, then tied to weights. The weights shown in Figure 1 are bottles of sand, hut any heavy object could be used. The tube is fastened to a ring stand with a clamp, while the ring stand is itself clamped to the lecture bench to prevent tipping. Use of the Bouncing Balls Model
Initially, two weights are dropped off the bench, causing two balls to bounce up to the end of the tube (Fig. 2). The weiehts cause the balls to stay as close t o one another as possible, giving the linear shape characteristic ot' tw
