Molecular Models for the Do-It-Yourselfer James P. Birk and John Foster Arizona State University, Tempe, AZ 85287
Since our collection of molecular models had become worn, and commercial models are expensive, we decided to develop a system for building models of molecular compounds. To meet our needs, the models must he easy to huild, inexpensive yet durable, and of asize suitable for large lectures of up to 250 students but also usable in small classes of 25-40 students. The system we developed has been used to huild a large variety of molecular models, including models of all the shapes predicted by the VSEPR theory. The svstem is easv to use. and models raneine in comulexitv from water to sucrose havk been built succ~ss~ully by indergraduate students. maduate teachine assistants. and facultv. Extensions of thl's system have be& used to build close-parked and ionic models an well. We have also taught this system to high school chemistry teachers, who have made numerous models for their classrooms (numbers have ranged from about 10 to 100 models per teacher during 8-hour workshops).
Table 1. Equlpmenl Used for Maklng Ball and Stlck Molecular Models Elecnic drill Drill guidgPortalign (with a round opening) Drill bit-% in. Reamer dowel-%-in. dowel with smoothly rounded edges Marker dowels-%-in. and '&in. dowels sharpened to a poim Double/Triple bondmaker-short Gin. and %&. dowels taped together brass welding md cut to appropriate lengths Painting wires-&in. Hole saw-mounted in a drill and used to make plywoad marking bases and drlll gulde bases Angle guides: Tenaheorai - poster board pyramd wrh holes in faces TI ang~lar-plywood marklng woe with hole in center Sq.are-plywood markmg oase vnh hole in center
Equipment and Supplles
The eouiument needed to huild models can he uurchased .. and assembled quite inexpensively. We use 2-, 2 5 , and 3in.-diameter Styrofoam halls to represent the atoms and7/16in. dowels to represent the bonds. These materials can he obtained a t reasonable cost a t art SUDD~V -.. and hardware stores. The necessary equipment is listed in Table 1. The drill guide (Fig. 1)is fastened to wooden legs or to a wooden base so the Styrofoam halls can be placed into the circular hase without contacting the workbench surface. For smaller balls, we insert a reducer ring made from an appropriately sized steam bath rine. The drill a i d e could also be screwed to a piece of plywoo2 and then,;sing a hole saw mounted in the drill. a 1.5-in.-diameter hole could he drilled into the center of tl;e plywood hase to provide the reduced opening. The tetrahedral angle marker (Fig. 2 ) , made of poster hoard, is based on one made of wood' and is found to he adequate as long as it is fastened together tightly around the ball with rubber bands. The pyramid has sides of 12.4 cm for 2-in. halls, 15.4 cm for 2.5-in. balls, and 18.7 cm for 3-in. balls. Small holes are placed in the center of each face, located at the confluence of the lines drawn from the three
' Trebella, J. J. Chem. 13 News 1983, Sept. (No. 142), 6.
Figure 1. (left) A Portaligndrill guide, win?elemic drill attached, is used to drill holes in Styrofoam balls. The Styrofoam balls are seated firmly in the base of the drill guide. The drill stop is set to give a hole with 2.5cm depth.
Figure 2. k i m l T h e tetraheha1 bond marker is made of Doster board on which foir triangles are marked. The poster board is folded aibng adjacent triangles and taped together, leaving one triangle as an enhy port.
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Number 12
December 1989
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corners t o the middle of the opposite edges. An alternative method of drilling holes a t the tetrahedral angles has been reoorted nrevi~uslv.~ ~ r i a n g h aor r sq;are angle markers (Fig. 3) are made of in. olvwood set on wooden lees. We used a hole saw to drill a h o c bf 2-in., 2.5-in., or 3-in- diameter in the plywood, deDendine on the size ball the marker guide was designed to accommodate. The wood base is marked from the center of the hole t o give guidelines for the necessary anales (90". 120°, and 1800). ' Atomlc Sizes and Bond Lengths
The sizes of the Styrofoam spheres used in our models were chosen to show approximate relationships in atomic sizes: first period-2-in. (5-cm) spheres; second period-2.5in. (6.4-cm) spheres; third period-3-in. (7.6-cm) spheres. The lengths of the dowels used for the bonds were computed using a 10-cm/A scale, with the bond lengths calculated from the centers of the spheres. A list of some useful bond and dowel leneths is included in Table 2. A 2.5-cm sink hole is drilled i n t o each Styrofoam sphere to seat the dowels firmly, so the dowel lengths listed in Tahle 2 include the 5.0 cm that will be inserted into the Styrofoam balls. Dowels should be sanded to give smooth ends; otherwise, the Styrofoam will tear and the dowel will likely not be perpendicular to the ball when it is inserted into the hole.
Table 2.
Molecular Model Bondand Dowel Lengths (Scale: 10 cmlA)
Bond
Bond Le?gth (Al
Band Lenglh (cml
Sum of Sphere Radii (cml
Dowel Showing (cml
Dowel Lenglh (cml
c--C
c=c
= Cr7c c--H CQ (alcohols, ethars) CQ (acids, esters) C--O C; 7c
Bond Angles
Distortion due to electron repulsion is ignored. Bond angles for sp3 hybridization in second period elements (C, N, 0 ) are represented as purely tetrahedral. Likewise sp2 bond angles are represented as purely trigonal. Marking Positions Before drilling, positions are marked using a marker dowel (or a painting wire) and the appropriate angle guide. Tetrahedral bond ~ositionsare marked bv laci inn the ball in a pyramid of the appropriate size andciosing;t snugly with rubber bands. A marker dowel is used to poke holes into the ball through the hole in the center of eaih tkce of the pyramid (Fig. 4). The ball is then removed and drilled. Other bond angles are marked with the assistanceof one of the marking bases. Two procedures arc possible. If the ball fits snuelv in the hole of the markine base (wirh the eouator of the b a i j u s t a t the top of the markyng base), paintingwires can be lined un with the anele marks on the base and inserted at the poskions to he dhlled (Fig. 3). Alternatively, one
hole can be drilled in the ball. A dowel is inserted into this hole. The hall is laid on the marking base with the dowel aliened on one of the anele marks. Another dowel of the same size, preferably sharpened, is laid on another angle mark and noked into the ball to mark that oosition (Fir. 3). That hole is drilled, two dowels are inserted into t h e bhl, and the orocedure is r e ~ e a t e duntil all holes have been marked and
-
-
Drilling Holes The Portalign drill guide has a circular hole in the base that will hold a 2.5-in. ball with a nice fit. (Wooden legs are added t o the base to give sufficient clearance for the ball.) A size reducer is used to hold a 2-in. ball, which will fall through otherwise. The guide is placed on top of 3311. balls
Tobey, S. W. J. Chem. Educ. 1#62,39,39
Fiaure 3. ilenl bases witha larw, hole are markedto ~rovidea marker . . Plvwood . guyde for angles otherthan tehahedral.ihin stiff wires can be'used to mark the position in which holes are to be drilled, using a marker base to determine the comct angles.
Fiove 5. ilefll An ahernatlve memod of usina tha marker base starts with on% haie drilled in'the ball, with additional holes being marked wlth a marker dowel of appropriate size.
Figure 4. (right) A d w dowel is used to poke holes into the Styofoam ball through the holes in tha faces of me tebahedral bond marker.
Figure 6. (right) Double and triple bonds are shown by two or three dowels inserted into adjacent holes, drilled in a straight line.
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Journal of Chemical Education
since they will not fit into the top of the hase. The ball is nlaced in the base of the drill guide (Fie. 1). This automati;ally centers the ball and ensuFes that tce hole will be drilled perpendicular to the surface of the ball. The drill stop should be set to give a 2.5-cm drillingdepth. Holes are drilled with a %-in. drill bit. A Ys-in. reamer dowel is then used to clean out the hole, widen the hole slightly, and firm up the Styrofoam structure surrounding - the hole. The %A-in.dowels will then fit snugly. Double and Triple Bonds
Since each bond is represented by a dowel, it is necessary .to drill two or three holes side by side for multiple bonds. The procedure and equipment described here will work only on 2.5-in. balls. althoueh " it would he nossible to modifv the equipment to work for other sizes as well. For consistency in representine the location of the electron clouds. a double hind should be drilled perpendicular to the plane'descrihed by the two single honds, and a triple bond should have its three holes in a straight line (Fig. 6). These holes are drilled with the drill guide and the doubleltriple hondmaker. The drill guide has two M-shaped protrusions designed to hold pipe for drilling. The hondmaker device is placed parallel to Gd touching the inside of one of the prot&ons-and lying on the base of the drill guide. When a hall is placed in the drillguide, it will beoffset from the renter by an appropriate amount to allow two or three hnlea to be drilled adjarent to one another (Fie. 7). If the small dowel of the hondmaker is placed toward 'the'center of the drill guide, the offset is sufficient to allow two adiacent holes. One hole should be drilled and the hall shouldbe rotated 180'. It is necessary LO makesure that the first hole isstill nernendicular to thedrill. Inserting a short dowel into the hol; may he helpful. The second hole is then drilled. Drilling holes for a triple hond is similar, except that the large dowel of the boidmaker is turned toward the center of the drill auide to provide a larger offset. To complete the triple hond, also d d l one normal hole at the center of the hall. Since this drilling procedure requires more skill than drilling single-bond holes, it is best to drill these holes first. The partially-drilled ball can then he placed on a marking base, with the holes located one
above and one below the base. If two dowels are inserted into the outermost holes, the hall will be centered on the hase. Painting wires can then he used to mark the other position(~)that must be drilled. Assembly of Model The model should he completely painted before final assemblv. A white glue such as Elmer's Glue-All works well to faste;the balls i d sticks together. If a bond should have free rotation, place glue in the hole, insert the dowel, twirl the dowel around to spread the glue, remove the dowel and allow the glue to dry. A hard sleeve of glue will provide a nice socket inside the ball that will protect the hall from damage during assembly and rotation. If a rotatable hond is not desired, the same procedure should he followed except that the dowel is not removed from the hole. Final adjustments to bond angles can he made readily while the glue is still wet. Simply "tweak" the dowel in the a~nronriatedirection. If the hole becomes enlarwd as a r&ult;additional glue can be added to fill the h o l e k c e the assemblv is com~leted.After dwine. " -. attempts to adiust bond angles will ;sually cause considerable damage t o t h e balls. Palnting Models Color Coding
Standard textbook atomic color codes are used for the halls: hlack for carhon, white for hydrogen, red for oxygen, blue for nitrogen, yellow for sulfur, and green for chlorine. Bonding dowels are usually painted some nonohtrusive color: tan for single, double, and triple honds, and orange for bonds with resonance character. Paints
We have developed three different schemes for painting our models. These differ considerably in ease of use, expense, and appearance of the painted models. In each case, we recommend painting the parts before they are assembled into the complete model. 1. Tempera and Shellac. Our preferred method of painting involves dipping the balls in a dispersion of tempera
Figure 7. (len) A doublelbiple bondmaker is laid on the base of the drill guide to offset the Styrofoam ball so holes can be drilled edjacem to one andher.
Figure 8. (center) Models pinted with tempera dispersed in shellac are attractive and durable.
Figure 9. (right) This model was painted with acrylic paints.
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Number 12 December 1989
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powder in shellac. Although tempera powder is normally dispersed in water, water-based tempera liquid does not cover Styrofoam well, since it does not wet properly, and the resulting surface is not very durable. However, if the tempera powder is dispersed in shellac, which used methanol as the solvent, the wetting properties, the adhesion, and the durabilitv are vastlv- imoroved. The balls can he oainted . either with a brush, or by spearing them with a painting wire and dio~ineinto t h e ~ a i n tIf. diooed. the excess naint should beallo~ed~odripoff, then the hall should twset'aside (on it3 wire) to drv. Drvine time isabout 1 b to the touch and about 2 h hardness;~o-get complete coverage of the Styrofoam, it is necessary to disperse about 225 g of tempera powder in 500 mL of shellac ('1%pound per pint). One pint of this mixture is sufficient to paint about 150 2.5-in. Styrofoam halls. The mixture must be shaken well before use since the tempera will settle out on standing. This painting method has generally given the most satisfactory results. Colors are bright, and the balls retain their roueh texture. eivine an attractive anoearance (Fie. 8). The surrace heco& ha& due to the sheiiac, givinga viry'durahle model. Paint is a ~ o l i e dto Stvrofoam balls exce~tionallv easily, with a short driing time. The cost of the paint is ve& reasonable. However, dowels are not painted easily. More than one coat is required, and coverage is not even. For this reason, we paint dowels with acrylics, latex, or spray paint, depending on the effect desired. 2. Acrylic Paints. Liquitex acrylic artist colors are somewhat exoensive hut are oackaeed in sizes (2 oz or 8 02) " -. convenient for modest projects. In spite of the expense, we use acwlic naints for some models. Colors are hrieht and the balls rekitheir rough texture, giving an attraccve appear-
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Journal of Chemical Education
ance much like that of tempera and shellac (Fig. 9). Dowels are painted easily with good coverage in one coat. Drying time is reasonably short (a few hours). However, the paint must be applied with a brush, and must he "poked" down into the structure of the ball surface, which is time consuming. The models are not as durable as those painted with tempera and shellac since the surface wears off with repeated handling. 3. L a t e A i n t and Spray Paint. Spray paints are easy to apply but have an organic base that dissolves Styrofoam. To use spray paint, it is necessary to paint the balls first with two coats (for complete protection) of inexpensive latex paint. Spray paints of appropriate color can then be sprayed overthe dry latex coat, provided any holesdrilled in the halls are protected by inserting short lengths of dowel. This method of painting is fairly inexpensive, particularly for small projects. The surface is very durable, hut it is smooth, so the attractive texture of the balls is lost. Either latex or spray paint can be used to paint the dowels. Althoueh the oaintiue is relatively easy, it is time consuming since three ;oats are needed. Any break in the latex coat results in a partially dissolved ball. Conclusion This svstem for buildine models has orovided us with a versatileset of useful models to use in our classes. Any new models that are required can usually be obtained within one or two days. An unexpected benefit has accrued when students areasked to build modelsfor us. Thesestudents Zenerally enhance their understanding and appreciation of the elements of structure and symmetry.
