Robert C. Olsen Pacific Lutheran University Tacoma, Washington 98447
Crystal Models
Just as pictures are superior to words, so are space models superior to pictures and the "right" model can be very meaningful both to the student and to the teacher. After searching for the "right" model to demonstrate crystals formed by metal bonding, we attempted one of our own. This type of model was first designed to demonstrate the number of particles in a crystal that can be assigned to a unit cell. The model (at left) is made to appear as a unit cell cut from the bulk of a large crystal; in the face-centered cubic system the model shows half a particle on each face and one-eighth of a particle on each corner t o give a total of four particles. To easily recognize the appearance of the face-centered cubic system, a model (right) was made to represent a portion of a crystal, one unit length on a side, but retaining all the particles whole. To show planes of particles, a model (center) was made using all the particles in eight unit cells. This model system allows particles to appear suspended in space without rods or bars as used in most crystal -
1 The cork halls were purchased from the Erno Products Co., 65 North 2nd Street, Philadelphia 6, Pennsylvania. The cement used hss the trade name "Dekophane", manufactured by Crystal Essenoe Corp., Plainfield,N. J.
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/ Journal of Chemical Educafion
models. One can readily see the five 111 planes which exist between alternate diagonal corners of the eightcell cube. The models are made of 1-in. cork balls,' cut and cemented to 1/16in. Plexiglas. The unit length is 4 in. The construction is simplified by making all joints cork to Plexiglas, with no attempt to join Plexiglas to Plexiglas. The eight-cell model is made with two faces omitted to allow the demonstrator to point to individual particles which reside within the crystal.
