A Visual Aid to Demonstrate the VSEPR Theory The Valence Shell Electron Pair Repulsion theory (VSEPR) is presented in most of the more recent chemistm books. Simnle and straiehtforward as this theow is, the author has found that many students, &tirulnrly iiwhmen, have a difiicuit time vi~ualiringthe structures in three dimensions and how they might come ~~~
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Fur yearsthe author told the students tovisualize a basketball and place a"pair of elertrons" on the surface. If another pair of electrons were then added to the haskGtball's surface, the theory predicted that they would cet ap far away from the previous pair as possible, or around on the back side. An imaginary basketball left aomethine to be desired and a real basketball could not be seen throueh and was too small for a large class. Also--how could you slide electrons wer the surface and keep them where YOU wanted them. pnrtirulnrlg if you had four pairs of ~lertrcms ~
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Thesolution to the prohlem i*shom in the ligure. It cunai~tsofa20in.dinmeter plarticglcd~~s h~amedi r m F:mlutw Transparent Glohea, 6OX7 Warrington Avenue. Philadelphia. Pa. 19143. (Big enough to see ~ h r w g h \. r l r;).). 11, hdtdlr,. It comes in t w hnlves. The halves were separated and a painted 5-in. diameter ityrofoam ball w*s held in plnc~in the center of the glohe by using a clear plastic support rod about '14 in. in diameter. On the outside of the globe the author glued small pieces of Velcro (%in.X Y2in.) at the places where he wanted trigonal, tetrahedral, square planer, octahedral, etc. configurations. The tetrahedral points (bottom 3) were positioned by making an equilateral triangle 0.80on a side and placing it horizontally inside of the lower half of the globe. Velcro was then placed where the 3 corners touched the surface. The author then obtained Kansas State University "Willie the Wildest" magnets to act as pairs of electrons. A piece of Velcro was glued on the back of these. They were sufficiently large to he clearly seen and yet easy to handle. The small pieces of Velcro on the side of the globe were hard for the students to see and even those in the front rows that could see them could not fieure out beforehand what they meant. The two halves of the globe were then taped together with transparent tape. Now it is easy to slide the electrons over the surface and have them stick where you want them to, and the students can clearly see the equal distance between the pairs. A few extra pieces of Velcro were used to show wrong situations. If too many of these are added then it gets confusing later an as to just which places are the correct ones. A single dot from a colored magic marker on each spot of Velcro allows the Lecturer to sort out the proper spots without the class being any the wiser. Far example, red = trigonal, green = tetrahedral, etc. The globes can be obtained in sizes from 6 to 30 in., with or without a stand. The cost ranges from $14 to $95 for the ones without stands. Small magnets were tried rather than the Velcro hut were unsatisfactory. The plastic wall is 3/~sin. thick, and this is enough that even 1's in. lh in. X 1in. magnet (Edmund Scientific Co) was not strong enough to hold the "electrons" in place under normal handling conditions. Kansas S t a t e University Manhattan. K S 66506
668 1 Journal of Chemical Education
Cliff E. Meloan
