Richard 1. Barren
New Mexico State University university Park
A Simplified Procedure for Making Structural Models
Numerous papers have appeared describing methods for making crystal structure and molecular models from styrofoam spheres. The purpose of the present paper is to describe a simplified procedure which requires only the simplest tools and practically no mechanical skill.' The jig used for marking angles is the simple device, made from scrap lumber and masonite, shown in Figure I . The top of the table is covered with a 4 X 6 file card with the most used angles (109.5', 120°, and 90') drawn in ink. As many jigs are needed as there are different sizes of spheres to be used, but they can be made in a few minutes. The sphere is placed in the jig which is made so that the equator of the sphere coincides with t,he surface of the table. Three-penny finishing nails are inserted into the sphere along the angle lines. To lay out the tet,rahedral angles, four nails are inserted along the 109.5' lines; and the sphere is then removed and replaced with the four nails in the vertical plane. The figure shows the sphere in this position with the last two nails in place. The two nails in the vertical plane which are closest to the camera were placed for reference only and will be removed. For molecular models it is desirable to cut facets on the spheres. The size of the facet is determined by laying out the desired internnclear distance to scale and then describing circles to correspond to the van der Waals radii of t,he atoms. The intersection of the circles determines the diameter of the facets. If no great precision in representing the van der Waals radius is required, stock sizes of spheres may be used. The spheres can be made smaller by rolling between two
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The Star Band Companv, Portsmouth, Virginia, is a source of supply for Styrofoam spheres, eggs, flat sheets, chenille, and cement. Plasteel Corporation, 26970 Princeton, Inkster, Michigan, offers molded polystyrene spheres in sizes from inches. Magnets may be obtained from Magnet Aids, I l l Marion Street, Patterson, New Jersey; and tempera, from Permanent Pigments, Inc., Cincinnati, Ohio. The hole cutter, used to make the jigs, is available from Montgomery Ward.
472
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Journal of Chemical Education
Figure 1 (Left). Jig with 3 penny Rnirhing noiir inserted into sphere to mark the tetrahedral angle. Two of the noilr in the verticol plone o r e for reference only and will be removed. Figure 2 (right). Tool w e d in cutting facets on spheres. Portion of sphere protruding through the hole is removed by obrosion.
hard flat surfaces. This increases the number of jigs needed but the jigs also serve as templates to gauge the amount of rolling required. I t is oft,en preferred to use a smaller scale for the van der Waals radii than for the covalent bond distance. To cut a facet, the sphere is held against a sheet metal plate having a hole of the dcsired diameter, as shown in Figure 2. The nail is centered in the hole and then removed. The portion of the sphere which protrudes is then cut off with a bread knife or removed with a coarse abrasive. Thin sheets of perforat,ed metal, known as "dragon skin" are very good for this purpose and are also useful for modifying spheres and making other shapes from st,yrofoam. Molecular models can he assembled by smearing the facets with white g h ~ e(Elmer's type) and pressing into place. Where facetsare not used, as in cryst,al models, the spheres can he held together by pieces of florist's chenille glued into holes in the spheres. If models with interchangeable ligands are desired, the opposing facets may be covered with Velcro (hook nylon) obtainable in sewing supply stores. Anot,her met,hod is to use small alnico magnets for this purpose. These magnets may also he inserted into the models in such a way as to simulate hydrogen bonding. The magnets are also useful for holding models in position on a steel-backed chalk board. Models should be painted with white shellac mixed with dry tempera colors.
