In the Classroom
A Novel Multipurpose Model Set for Teaching General Chemistry
W
H. O. Gupta* and Brahm Parkash National Council of Educational Research and Training, New Delhi, India
Uses of atomic, molecular, and crystal models in research and teaching of chemistry are described from time to time in this Journal (1–14). To our knowledge, no single model set is available that is capable of demonstrating a majority of the structures in general chemistry courses. We report here a low-cost and unique molecular model set capable of generating a large number of structures for teaching and learning general chemistry. Description of the Model Set The model set consists of 11-hole balls (3.0-cm diam), 4-hole balls (1.5-cm diam), straight long and short connectors (Y-type, T-type, L-type), trigonal bipyramidal (5-way) connectors, lobes, plastic tube pieces (1.5 cm diam), and sleeves. All components of this set are plastic injection-molded from low-density polyethylene, except tubes and sleeves, which are cut into suitable sizes from commercially available polyvinyl chloride (PVC) tubes. One of the most important components of the kit is the 11-hole ball. The arrangement of holes in this ball is shown in Figure 1. One hole is obtained in the molding process and we designate it hole 1; the rest of the holes are drilled. First, holes 2, 3, and 4 are drilled in tetrahedral positions with respect to hole 1 with the help of a tetrahedronshaped jig. Then, using a hexagonal jig, hole 5 is drilled at the circular line dividing the ball into two halves near hole 4, and Figure 1; Arrangement of holes 6 through 10 are drilled holes in the 11-hole ball. on this line 30°, 90°, 150°, 180°, and 270°, respectively, from hole 5. Finally, hole 11 is drilled opposite to hole 1. All atoms other than hydrogen (represented by 4-hole balls) are represented by the larger 11-hole balls, which are colored according to the international color code to represent atoms of different elements. The connectors include long connectors (rods 5.5 cm long to represent bonds), short connectors (2 cm long to facilitate close contact of the balls), T-type, Ytype, L-type, and trigonal bipyramidal connectors. The last four types are also used in conjunction with PVC tube pieces cut to the required lengths to depict special features such as multiple and multicenter bonding. Additionally, these PVC tube pieces represent delocalization effects. Small colored sleeves, which can fit over the long connectors, are used to show spatial positions of the bonds. extended version of this paper appears on JCE Online at http://jchemed.chem.wisc.edu/Journal/issues/1999/Feb/ abs204.html. W An
*Corresponding author. Email:
[email protected] 204
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Figure 2. (a) Model of four CABC closest packed layers. (b) Face-centerd cube (FCC) is recognized readily by rotating the assembled balls.
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Advantages The 11-hole ball gives tetrahedral, octahedral, trigonal, trigonal bipyramidal, and square planar symmetries. The arrangement of holes enables the user to generate a large number of structures with a minimum number of types of balls. With the help of the connectors, it is possible to (i) assemble the balls in either skeletal form or semi-space-filling form, (ii) adjust the length of the bonds, and (iii) visualize delocalization, multiple, and multicenter bonding. Flexibility of the long connectors allows students to see the strained structures and at the same time provides enough rigidity to hold the large structures. Closest-packed structures, octahedral and tetrahedral holes, and semi-space-filling models of diamond, ZnS, CaF2, etc. can be shown using multisymmetry balls and small and trigonal bipyramidal connectors. Figure 2 shows the equivalence of cubic closest packed (CCP) and face centered cubic (FCC) structures. Besides common structures in inorganic chemistry, pπ –pπ and pπ–d π. bonding as in Cl3PO (Fig. 3) and three center–two electron bonding as in the borane structure can be easily demonstrated. This model set is also able to demonstrate stereoisomerism, strained bonds, spatial bonds, delocalization (Fig. 4) in organic molecules. To sum up, the model set is multipurpose—capable of showing a majority of structural features in crystal, organic, and inorganic chemistry with a minimum number of balls, thus reducing the cost considerably. We have produced only a few sets. Interested readers can develop the set if machine shop facilities are available; it could also be commercialized.
Journal of Chemical Education • Vol. 76 No. 2 February 1999 • JChemEd.chem.wisc.edu
In the Classroom
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Figure 3. Model of Cl3PO structure. The orbital overlap in forming a p π–dπ bond between O and P is represented by a suitable arrangement of connectors and PVC tube pieces.
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Acknowledgments We would like to acknowledge the help of a number of experts, including A. C. Handa, P. S. Jaiswal, S. C. Datta, A. K. Singh, and R. K. Bansal. We also thank the technical personnel of the Department of Computer Education and Technological Aids, NCERT—particularly, Virendra Kumar, Balbir Singh, S. C. Sharma, and M. A. Khan. Literature Cited 1. Mislow, K. Introduction to Stereochemistry; Benjamin: New York, 1965; pp 42–46. 2. Petersen, Quentin R. J. Chem. Educ. 1970, 47, 24. 3. Bent, Henry A. J. Chem. Educ. 1984, 61, 774. 4. Walton, A. Molecular and Crystal Structure Models; Halsted: Chickester, England, 1978. 5. Gordon, A. J. J. Chem. Educ. 1970, 47, 30. 6. Adler, A. D.; Steels, W. J. J. Chem. Educ. 1964, 41, 656. 7. Sanderson, R. T. Teaching Chemistry with Models;Van Nostrand: Princeton, NJ, 1962. 8. Vogtle, F.; Goldschmitt, E. J. Chem. Educ. 1974, 51, 350. 9. Kildahl, N. K.; Berka, L. H.; Bodner, G. M. J. Chem. Educ. 1986, 63, 62;
Figure 4. (a) Model of chlorobenzene. Annular rings of PVC tube pieces on C skeleton show delocalization of pi electrons. (b) Model of benzy1 chloride. A lobe represents a lone pair of electrons
10. Birk, J. P.; Coffman, P. R. J. Chem. Educ. 1992, 69, 953. 11. Brumlik, G. C.; Barrett, E. J.; Baumgarten, R. L. J. Chem. Educ. 1964, 41, 221. 12. Barrett, E. J. Chemistry 1967, 39, 40. 13. Barrett, E. J. J. Chem. Educ. 1967, 44, 146. 14. Jensen, W. B. J. Chem. Educ. 1989, 57, 637.
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