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A NEW CRYSTAL MODEL CONSTRUCTION SET F. C. BRENNER Polytechnic Institute of Brooklyn, Brooklyn, New York
TEE IN~TRUCTOR of inorganic chemistry who wishes to emphasize structure has been handicapped by the lack of a crystal model set. The author has developed a versatile crystal model building set from a child's construction toy, aptly called Molly Kewls. The set is made of rubber halls and cellulose acetate tubes. The balls are of two types. Each ball has six molded holes octahedrally placed. The other type of hall has been prepared by drilling holes in the molded balls to obtain another geometrical arrangement. This type has fourteen h o l e s s i x octahedrally arranged and two sets of tetrahedrally arranged holes. The balls are red, blue, or yellow. The cellulose acetate rods are the same color. Straight pieces are cut to any desired length by scoring with a knife and break'mg. Curved pieces are used to form double bonds and illustrate Werner complexes. Odd-angled bonds are formed by bending the rods in hot water. According to the Hume-Rothery classification twentyone of the elements crystallize in one of three lattices; the hexagonal close packed, cubic close packed, and
Figuxs 2.
Fluorite Type Lattice
the body centered cube. One of these is illustrated in Figure 1. Construction is facilitated since one of the holes passes completely through the ball. The two types of alloys may be shown with these
Figure 3.
"YL..C
Figura 1.
Cmbi. 010-d
P=sked Lattis- Showme Hexagonal Close
packad L ~ Y . F
IIIYUCIU.
. i
U." '.
Mirror 1m.g..
.LL'"lY.Y
"I
UYYU
yWYYU
1 1 .
Yll"
spaces between the balls will illustrate the interstitial type. Replacement of some of the balls in the lattice 371
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JOURNAL OF CHEMICAL EDUCATION
Figure 5.
Fisure 4.
Phosphorova Pentorids IPlOjo) Molecula
with balls of another color will illustrate substitutional type. Figure 2 represents the fluorite type crystal. The unit cell is outlined in dark tubimg with the bonds in light tubing. Figure 3 shows a pair of mirror images of a Werner complex ion, cobaltic dichloro-diethylenediamine ion. The central ball represents the cobalt, the two light balls the chlorine, and the balls connected by the dark arcs represent the ethylenediamine molecule. Rotation of these models show that thgy are not superimposable. Inorganic molecules such as phosphorus, phosphorus trioxide, and phosphorus pentoxide (Figure 4) clearly illustrate the molecular nature of these substances in the vapor phase and their proper molecular formulas. The spatial arrangement of atoms in simple organic compounds may be shown. The models have free rotation about single bonds. The ring structure of benzene is shown in Figure 5. The construction of a model of a strained cyclic
Benzene
compound shows that the geometry of the molecule is not such as to permit easy ring closure. The rubber construction of the balls allows the ring to be closed by the application of force (Figure 6). The assistance of Mr. Richard Hopkins in preparing the model set, and of Paul Bonhop, Inc., and Mr. Machinist in furnishing the prinls is gratefully acknouledged.
F'igure 6.
St~ainedRing Formation
