In the Classroom edited by
cost effective teacher
Hal Harris University of Missouri–St. Louis St. Louis, MO 63121-4401
Use of Pom Pons To Illustrate Cubic Crystal Structures Susan G. Cady P. O. Box 14176, East Providence, RI 02914-4176 In general chemistry classes, students are introduced to the ways in which atoms are arranged in cubic crystal structures. Transposing the textbook illustrations into three-dimensional structures is difficult for some students. This transition is easier if a three-dimensional model is available for examination. Models constructed from carved balsawood (1), cut and polished Plexiglas (2), wires soldered to an iron center (3), and reflective glass boxes containing light-emitting diodes (4) can demonstrate several or one specific basic crystallographic unit. A substitute for expensive commercial crystal cage models, which requires less preparation time and dexterity than the above crafted models, is a model constructed of Styrofoam balls and toothpicks. Here Styrofoam balls representing atoms are associated with each lattice point in the unit cells. Similarly, cork balls glued to Plexiglas (5) and marbles that rest in holes cut in stacked Plexiglas shelves (6) have been used to represent atoms. Like the marble model, a cardboard box “theater” model with interchangeable cellophane screens (7) can be constructed to effectively demonstrate the relationship of several neighboring unit cells for different cubic crystal structures. In my general chemistry classes, I have utilized olefin pom pons1 to illustrate different cubic crystal structures (Fig. 1). Unlike Styrofoam balls that are difficult to paint
or not always readily obtainable in a variety of colors, pom pons come in assorted colors. These durable pom pons also are easier to store without breakage. Assembly does not require special cutting skills or long preparation times. The olefin material is coarse enough to allow the pom pons to be stacked upon one another or be positioned side by side without rolling off the table. No glue is needed, so the pom pons can be reused or rearranged to make additional crystal structures. For added stability, it is easy to sew the pom pons together. An additional advantage of this pom pon model over most of the previously described models is its cost. Bulk quantities of pom pons are inexpensive. Therefore the teacher can provide each student with enough pom pons to build models for all three of the basic cubic crystals. To demonstrate how the atomic radius will vary when comparing the different types of cubic crystal unit cells, students can use different sized pom pons (Fig. 2). Consideration of the size of the atomic radius for a given crystal structure is important when students are asked to calculate the density of a crystal. Students can also use the pom pons for hands-on examination of different packing arrangements (Fig. 3), such as hexagonal close-packed and cubic close-packed structures.
(a)
(b)
Figure 1. Using the same sized pom pons, an increase in the overall physical size of these crystals is illustrated as one proceeds, (left to right) from the simple cubic structure to the body centered cubic structure to the face centered cubic structure. Photo by John B. Zibluk.
Figure 2. To keep the edge length for each type of filled cubic cell constant, the radii of the atoms must be different. This can be made visually apparent if different sized pom pons are utilized to construct each type of filled cubic cell. Photo by John B. Zibluk.
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Figure 3. Different packing arrangements can be demonstrated using different colored pom pons. (a) Triangular layers of pom pons can be arranged (or sewn together) to show how these planes can be stacked with loose pom pons to create hexagonal closepacked and cubic close-packed structures. (b) A loose pom pon can be placed on the top and the bottom of two oppositely pointed (antiparallel) triangular planes to produce a face centered cubic structure. Photo by John B. Zibluk.
Journal of Chemical Education • Vol. 74 No. 7 July 1997
In the Classroom Note
Literature Cited
1. Pom pons style #3535 Herculon* (* registered trademark of Hercules, Inc., for its olefin fiber), manufactured by Maxwell International, Chicago, bar code 29894 35350, 100% olefin pom pons ca. 300 to a bag, assorted sizes (1/2”, 1”, and ca. 1 1/2”) and colors, purchased at a hobby store for less than $10.00.
1. 2. 3. 4. 5. 6. 7.
Li, T.; Worrell, J. H. J. Chem. Educ. 1989, 66, 73. Slabaugh, W. H. J. Chem. Educ. 1959, 36, 288. Brady, K. T. J. Chem. Educ. 1983, 60, 36. Kennard, C. H. L. J. Chem. Educ. 1979, 56, 238. Olsen, R. C. J. Chem. Educ. 1967, 44, 728. Kildahl, N. K., Berka, L. H.; Bodner, G. M. J. Chem. Educ. 1986, 63, 62. Komuro, Y.; Sone, K. J. Chem. Educ. 1961, 38, 580.
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