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A Device for Making Classroom Molecular Models Bruce Mattson Creighton University, Omaha, NE 68178 Large classroom molecular models greatly enhance students' ability to conceptualize formulas i n terms of the molecules that they represent. Large models on display (possibly hanging from the ceiling) also enhance the appearance of the chemistry classroom. Unfortunately, large molecular model kits are expensive and are not designed for creating permanent exhibits. Many teachers produce their own large molecular models from Styrofoam balls and wooden dowels. The major problem with this approach is that good tetrahedra are extremely difficult to make. I have designed a device (Fig. 1) for making high quality tetrahedra suitable for use a s classroom exhibits. The device is easy to use and a good tetrahedron can be produced in less than a minute. The device is built from ordinary materials that cost just a few dollars. Construction requires readily available power tools and takes about 2 h to complete.
ter for the central atom and 2-in. (5-cmjdiameter far the terminal atoms 5 bolts ('I4-in. (6-mm) diameter) with appropriate nuts. The sugested leneth - of the bolts is '/.An. (12 mm) woad & ootional: washers (See S t e 8) ~ I 6 m m dowrls :,re nkrdpd in ordrr to makp hods teach bond typwally wll use n 8-1" 20 e m , length of dowel Cost of Materials Approximate cost is $ 3 4 (for the model-makers only; materials for the models-Styrofoam, dowels and painkare not included in estimate.) lbols Needed table saw with '/,-in. (3-mm)carbide blade and a set of dado blades (to cut 31s-in. (or 9-mm)grooves) power drill with a bit (suggested 'I4-in. (6-mm)) table scroll saw adjustable wrench screwdriver appropriate for the bolts 'lEme Required for Construction 2h
Figure 1. The assembled Tetrahedron Maker. A second device has been designed to produce 90' and 120 "bond angles in any combination. Together these two devices can be used to create any molecular structure encountered in a n introductory chemistry course. I recommend that the central atoms be constructed from 3-in. (8cm) diameter Styrofoam balls available from craft and hobby stores. (Some "dime" and sewing stores also carry these.) Hydrogen atoms are represented with 2411. (5-cm) balls. The balls can be painted prior to final gluing. Bonds are made from 'Id-in. (6 mm) wooden dowels.
Building Your Classroom Molecular Model Makers Materials Needed one square foot (approx. 30 cm x 30 cm) of 'Is-in. (3 mm) hard board (Masonite or equivalent) a 16-in. (41 em) length of 'I2 x 6-in. (approx. 1.3 x 15 cm) pine (the 6-in. dimension is lumber yard jargon for a 5.5-in. width of lumber that was once &in. wide before finishing). This piece must not be warped. Styrofoam balls (available from craft or bobby stores, party stores, "dime" stares, ete.), suggested 3-in. (8-cm)diame-
Figure 2. Plan for the vertical portion of the Tetrahedron maker. Volume 71
Number 11 November 1994
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Constructing the Tetrahedron Maker
1. Start by obtaining several of the Styrofoam balls that you plan to use in making tetrahedral classroom molecular molecules. These plans call for the use of 3411. (7.6-cm) diameter Styrofoam balls. If you choose a different size, use a compass to modify Figure 2 by drawing a circle with the same radius as that of the balls that you are using. 2. Cut out two 6 x 8 in. (15 x 20 cm) pieces of hard board using the table saw. 3. Trace the plan shown in Figure 2 onto the two pieces of hard board. Indicate with some sort of mark the lowerleft corner of each sheet of hard board. The details of the plan (Fig. 2) given on the inside of the V-shape (pieces B and C) need to be traced onto only one of the two hard board sheets (two of the pieces labeled "A" are needed and only one Piece B and one Piece C are needed.) 4. Use the table saw to cut out the V-shape. Cut right down the center of the line so that Pieces A and B (or Aand C ) contribute equally to the material lost by the blade cut. 5. The table saw also can be used to finish cutting out Pieces B and C; however, these last two cuts should be made so that the blade does not remove any material from Pieces B or C (Fig 3). 6. Use a table scroll saw or a coping saw to cut out the circular arcs of both of the A pieces and of the B and C pieces. 7. carefully align the two "A" pieces. The base and the V-shapes must be very carefully aligned a t this point. While the two A pieces are being held firmly together and perfectly aligned, drill a '/,-in. (6 mm) hole through the two pieces. The appropriate location for these holes is marked with "+" on the Template (Fig.4).
,blade position of blade to cut Piece B
Figure 3. proper blade position to cut pieces Band C.
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Figure4. Position of '14-in. drilled holes.
Figure 5. Position of spacers. 978
Journal of Chemical Education
I/8-inch dado
I
I u 3/8-inch dado
Figure 6. Positions of the dado cuts in the base of the Tetrahedron Maker.
Figure 7. Left The assembled Tetrahedron Maker. Right: Styrofoam ball in place and ready to be fined with bonds. 8. The two A pieces will be bolted tozether with a 'la-in. (3 nim, spacer hctwccn the two piwrs. (.'sing u set ofwashers that has a conihlned thickness of approximately h-in is ideal. Alternatively, holes can be driiied through scraps of hard hoard.
'
CAUTION: Never hand hold small pieces while drilling. Use a vice or vice grip to hold the small pieces. Fasten the two A pieces, separated by the 'la-in. spacer together. 9. Cut two small rectangles of hard board (approx. 1in. x 3 in. (3 x 8 em) to serve a s spacers. Glue these spacers between the two Apieces and position in the two corners of the assembly so that the long side is aligned with the base and the short side is aligned with the sides (Fig. 5). 10. Cut the 16-in. (41 cm)piece of pine into two pieces of approximately equal size. Cut a 3/a-in. (9 mm) dado along the length of the 8-in. piece. The depth of the dado cut should be 'I4-in. (6 mm) into the pine and should be centered as per the left figure (Fig 6). Next cut a 118-in. (3 mm) dado perpendicular to the Va-in. cut and also center along the length a s per the right figure above. This second dado must have the same depth into the pine as did the first dado cut. 11. Glue the V-shaped assembly (the A pieces) into the 31s-in. dado. Work the assembly down into the groove until it is flush with the base of the cut. Glue Piece B into the '/&in. dado cut and flush up against the A assembly (the arc of Piece B should be facing up and toward the middle.) DO NOT glue Piece C into place. I t must he removable. 12. The Tetrahedron Maker is now complete. Figure 7 shows the assembled Tetrahedron Maker (left); and a t right, a Styrofoam ball is ready to be fitted with bonds! Use of the Tetrahedron Maker 1. Cut four dowels into %in. lengths (20-an) for each tetrahedron to he constructed. 2. Remove Piece C and place the Styrofoam ball into the Tetrahedron Maker. CYou may want to paint the balls prior to this step.) Typical Color Scheme
Hydrogen - white
Sulfur -yellow
Carbon - black
Chlorine -green
Nitrogen - blue
Oxygen -red
3. Return Piece C back to its proper position. The Styrofoam ball is now securely held in place. 4. Place the first dowel along one of the guides (grooves) of the V-shape. Slide the dowel along the guide and push the dowel into the stryofoam to a depth of approximately l-in. (3-em). While holding this dowel in the guide, insert the second dowel by using the other half of the V-shape a s a guide. If you are building a bent molecule such a s water, you can go to Step 6. 5. While holding both dowels in their respective grooved guides, insert a third dowel into the Styrofoam by guiding it along the angled edge of Piece B or C. Make sure that the
dowel remains in the plane of Piece B or C. The fourth dowel also can be inserted a t this time. 6. Remove Piece C and the Styrofoam ball. 7. The dowels and balls should be glued in place with generous portions of wood glue. 8. Smaller halls can be added to represent hydrogens, etc. Constructing the Trigonal Maker Plus
1. Refer to Figure 8. Start by cutting the four 318-in. (9 mm) dado cuts in the the 8-in. (20-cm) piece of 'I2 in. x 6-in. (1.25 x 15 cm) pine. The depth of the dado should be 'I4-in. (6 mm) into the thickness of the pine and should be positioned a s per Figure 8. (The dado cuts in your construction will possibly extend past the intersection of the three trigonal lines.) 2. Next, use a compass to trace a circle centered a t the intersection of the three 120" cuts. The radius of the circle must be equal to that of the Styrofoam balls. Draw a second circle centered a t the intersection of the two perpendicular cuts (the circles will probably overlap) (Fig. 9). 3. The three guides that are made next must have the proper dimension so that when they are in use, the top of each guide is exactly one radius above the plane of the pine. Here's how. Measure the depth of the dado cut (suppose it is '14-in.). If you are using 3-in. diameter Styrofoam balls, the guides must have a n after-cut dimension of '14-in. + 1 'Iz-in. = 1314-in.The width of the guides should be approximately 1in. (2.5-em.) Cut out six of these guides. Cut out three hard board spacers that are the same width as the guides and a t least 'I4-in. shorter than the guides. The guides and spacers will fit into t h e dado cut a s per the side view shown in Figure 10. 4. Carefully align the three pieces a s per the figure above. Drill a 'I4-in. (6-mm) hole through the three pieces. Fasten with a bolffnut. Instructions for Makina All of the Commonlv ~ncountered-~eometries in lntroductorv Chemistrv Courses (The tetrahedron must bemade with the Tetrahedron Maker.)
Geometry:
Instructions:
Place guides at D. E, F. Insert dowels along E and F trigonal plane (sp2) Place guides at A, 6,C. lnsert dowels along A, 6,and C. trigonal pyramid Place guides at D, E, F. lnsert dowels along D. E and F. Remove the dowel from Position (dsp3)(also distorted tetrahedron F. Move the three guides to positions A, 6, (see saw) and T- and C. Place the dowel from Position D in shaped) Position A. Direct the dowel from Position E perpendicular to the plane of the pine board (straight up). Install dowels at Positions 6 and C. Square pyramid Place guides at D, E, F. lnsert dowels along and octahedron D, E, and F. Remove the dowel from Position F Direct the dowel from Position E (dsp3) perpendicular to the plane of the pine board (straight up), insert dowels at Positions E and F. This completes square pyramid. Complete the octahedron by rotating the Styrofoam bail 90 about the vertical dowel. insert the sixth dowel into Position E or F. Sq~areplane Place gu des a1 D. E. F. nsen dowe s a ong D. E, and F Rotate Styrofoam bal oy 90 ' ano nsen me fourin dowe M o e c ar ~ model marers can be p x n a r e o from the amor Reqrssl oraer forms from the author at the address given or eMail your request to the author at
[email protected]. linear (sp)
Figure 9. Position of the two circles for the Trigonal Maker Plus.
Figure 10. Side view of a Trigonal Maker Plus guide.
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6. The Trigonal Maker Plus is now complete. Use of the Trigonal Maker Plus
Figure 11. Dado cut labels used in the instructions for model construction. 5. Prepare two more guide assemblies a s per Steps 3 and 4. None of the guides will be glued into the dado cuts.
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Journal of Chemical Education
1.Cut 'I4-in. (6-mm) dowels into 8-in. lengths (20-cm) for each bond that you wish to make. 2. Position the guides in the Trigonal Maker Plus. To make a trigonal plane (sp2), position the three guides in the dado cuts labeled A, B, and C as shown in Figure 11. The edge of the guides facing the intersection of the three dado cuts should be positioned a t the inscribed circle. 3. While holding both dowels in their respective grooved guides, insert a third dowel into the Styrofoam. 4. The dowels and balls should be glued in place with generous portions of wood glue. 5. Smaller balls can be added to represent hydrogens, etc.
