overhead projector demonstrcr tions
edited by DORISKOLB Bradley University Peoria, lL 61 625
A Magnetic Two-Dimensional Analogue of VSEPR C. Frank Shaw Ill and Bryan A. Shaw The Universitv of Wisconsin-Milwaukee Milwaukee w;, 53201 Valence shell electron pair repulsion (VSEPR) theory is animportant teaching tool. Its principle is straightforward, and it uses simple rules to successfully predict the structures of many simple compounds of the basic formula where A is the central atom: B is a bonded atom: and E is alone pair (13).R.ecently it was shownthat arandom walk of noint charws on the surface of a snhere can replicate simple structires (n = 2 4 ; rn = 0) and predict higher-order (n = 7-50) structures (4). We made a serendipitous discovery while exploring the shanes of maenetic fields that were generated by maenets of ;arious sGpes. Magnets that are shaped like d i k s or washers will renlicate. in two dimensions. the repulsions among electronbairs that underly v ~ ~ ~ ~ t h e o r y : Three, four, or five "terminal" magnets with their south poles up are brought near a "central" magnet with its north pole up. They spontaneously arrange themselves about the central "atom" to form triangular, square, and pentagonal arravs. (See Fie. 1.)The north-south attractive force of the ma*ets mimics the bonding force between a central atom and its terminal atoms. The mutual renulsions amone .. the 2-5 south-up magnets mimic the electron pair repuls~ons of VSEPR theory. The outer mamets telerrmn patrs, roll about the centrai magnet (atom);ntil they find the configuration of minimum energy. Spontaneous rearrangements from trigonal planar to square planar to pentagonal planar occur when the fourth and ffth south-up magnets are introduced. These are particularly striking. Magnets of two sizes were tested (45-mm o. d. x 20-mm i. d. x 3-mm thick; 31-mm 0.d. x 12-mm id x 6-mm thick). The smaller, thicker ones were better. This demonstration can be carried out before small groups a t the laboratory bench or before a large group using an overhead projector. Different colors of transparent acetate can be placed over the centers of the washer-shaped magnets to show which pole is up and to distinguish between "central" and "bonded" atoms. They also make the demonstration more bright and wlorful. Keep the magnets free ofmagnetic debris to minimize friction. This facilitates then spontaneous rearrangements. In principle, two magnets should he linear about H central one, but the potential well is too shallow for precise alignment; six south-up magnets are closed packed about the central magnet and form a t best a metastable arrangement. This Journal (5) has previouslv. reported a magnetic . model using iron filings to demonstrate the relative s i z e of bonding and nonhmding electron pairs. In our system, the distortions induced hv nonbondine oairs and multinle bonds can be simulated A t h magnets%at are larger than Correspondence should be addressed to C. Frank Shaw.
Two-amensional magnetc representattons of AB,. AB,. AB,, ana AB,E Tne nonn-so~tnoesgnatton ndcares when lace is ~ pTne . arger magner represent ng me one par (E n AB3E names a s~otle but distinct distortion from the square planar geometry of AB,. those used for the singly bonded atoms. The effect is best seen in the planar representation of AB8E. Literature Cited
4. weinrach,J.B.;Carter, K.L.:~ m ~ . tD. t ,W.; MeDouell,
H.K J Chem. Educ 1880,
67,995-999. 5. Schobert,H.H. J. Chem. Edvc 1973,50,651-652.
A Magnetic Illustration of the VSEPR Theory Manuel Hervas and L. PhilliD Silverman Vrg ma Polytecnnc nsr rJte ana.srare J n versny B acdsoug VA 24061 The Valence Shell Electron Pair Repulsion Theory is very useful for General Chemistw students. With it students can learn to predict the shapes of molecules and some other imnortant nronerties of molecules such a s polarity. I t is wokhwhileto illustrate this theory in the classroom, so we have develoned a simnle method to show the predictive powers of ~ S E P Ron'molecular shape using tyrof foam balls, magnetic bars, and a n overhead projector. Materials two 3-in. Styrofoam balls' four 5/16 x 1.25-in.magnetic stirring bars one transparent tray one 114-in.0.d. cork borer 'Note : Disksof Styrofoam or cork may work bener than balls, since the hemispheres haveatendencyto rollover especiallywhenstronger magnets are used. Volume 68 Number 10 October 1991
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Cut each ball in half. Bore a l-in. hole into the center of each of the flat faces created bv cutting the balls in half. Insert the magnets Into the hol&, payingcareful attention to have three of the hemispheres with the same mametic orientation (A). These three hemispheres are the p&phera1 atoms. The hemisphere with the magnet in the opposite orientation is the central atom (B). Care should be taken to make sure that the magnets are secure in their hemispheres. If necessary use an adhesive that will be impervious to water. Place the transparent tray on the overhead projector, and pour into the tray about 1in. of water. Place B and two A's
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Journal of Chemical Education
into the water. B will eventually attract both of the A's, which represents bond formation; and both of the A's will be as far apart from each other as possible, which represents repulsion ofthe two negative centers. This represents a linear molecule. Three of the A's will illustrate a trigonal planar molecule with bond angles of 120°. Using this same idea, it is not difficult to develop other magnetic analogies of electrostatic phenomena such as dipole interactions between molecules. For example, the Thomson model of the atom can be illustrated by using a bunch of small balls with magnetized pins as the electrons in a Petri dish for use on an overhead projector.
