Coffee-Stirrer Structures Robert Becker Kirkwood High Schwl, Kirkwood, MO Structural models serve to clarify the difficult concepts of form and symmetry. Students henefit by viewing these modunderstanding increases even more els, and the stud&' when they construct the models for themselves.' As a result, students are encouraged (and often required) to purchase model kits for college chemistry courses. Structural models are also very helpful to students in high school chemistry. However, the high price of commercial model kits can be prohibitive for a tvpical high school class. Students need a n &expensive and versatile s&tural model kit thevcan use and re-use to buildmolecular and crystalline structures.
A One-Blt Solution
With this in mind the author recommends a very simple solution: coffee stirrers and hook-up wire! The coffee stirrers must be of the "double-barreled" variety (two thin straws fusedside by side: "Spir-it" brand, item "C-1," for example), and thev mav be left full-leneth or cut in half. The wire should b e inshated and of aga&e that fits snugly inside the coffee-stirrer holes. (Solid 22 r a from Radio Shack seems to work, though it obviously depends on the hole size of the coffee stirrers ourchased.) ~ o n s t r u c t i o nis quite simple: v-shaped segments of the wire (3-4 cm in length) can be used to create friction-fitted junctures involving any number of stirrers. The figure illustrates the assemblv of a three-way juncture that, depending on the angles made, can serve as atrigonal planar, trigonal pyramidal, or T-shaped geometry or as the vertex of a cube, tetrahedron, or prism. Because the stirrers all have two holes a t each end, virtually any juncture, polyhedron, or lattice network can be constructed. Furthermore the flexibilitv of the wire enables one to construct a cube, for example, and then auite easilv reshaoe i t into a rhombohedral. CNB-dodecahedral or square antiprismatic configuration. And because the materials are all waterproof, thev can also be used to construct geometric frames for soap film investigations: Ideallv. each student would be issued at the beginning of the schoil year a small Zip-Loc bag containing 15-20 stirrers and 25-30 segments of wire (approximate total cost per kit: $0.12!). With holes punched in the upper corners of the bag above the zip-seal, this kit could easily be kept in the student's loose-leaf notebook, ready for use a t any time. ~~
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Some Suggested Model-Bulldlng Homework Exercises Illustrate General Concept of Molecular Geometry Instruct the students to construct a three-way juncture, spread the stirrers apart as far as possible, and use protractor to measure the angles created. Then have them repeat it far a four-way juncture. Although the students should all be able to get the trigonal planar arrangementwith its 120' angles, many will assume the most spread out four-way junction is the square planar configuration. Some will discover for themselves, however, that a nouplanar arrangement (such as tetrahedral) can beat out square planar by nearly 20°! Once everyone is aware that they arenot confined to two
Coffee-stirrer assembly fara typlcal Mree-way juncture.
dimensions,assign the five- and six-wayjunctures, and see what the students come up with.
Introduce Cyclic Hydrocarbons Have the students construct four., five-,and six-membered rings, all with bond angles of approximately 109'. They will come to appreciatefor themselves why four-carbon rings are so strained and unstable, why five-carbon rings are more or less flat, and why rings of six carbons or more must be "puckered out of the plane to accommodate the 109" angles. (See how many come up with the "chair" configuration for cyclahexane and how many come up with the "boat".)
Discussing Network Covalent Crystals Have each student construct and bring in a 3-dimensional structure in which every lattice point is tetrahedrally bonded. See how man" build the structure of diamond (containineall "chair" confieuratkn C6 rings) and bow many b"ild the lokdaleite structure (containingsome "chairs" and some "boats"). Both will probably be built in more or less equal numbers, and both are correct solutionsto the problem, hut lonsadaleite is considerably less common than diamond."lso, heve the students construct and bring in layers of graphite, and show them how these layers can be stacked in the ABAB . . .sequence to make alpha- (or hexagonal) graphite or in the ABCABC . . . sequence to make beta- (or rhombohedral) g r a p h h 3
Other Organic Applications Have the students try ta huild various organic molecules. See what methods they develop for representing halogens, functional groups, and multiple bonds.
' Walton, A. Molecular&C~stalStructureMode1s:Horwood:1978.
Boys. C. V. SoapBubbles, Their Colours and fhe Forces Which Mold Them; Dover: 1959: p 90. Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements: Pergamon: 1984; pp 303-305. Volume 68 Number 8 June 1991
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