James C. Howald and Gerald D. Smith Huntington College lndiana,46750
Crystallography-A January Term On the Properties of Crystals
A Januarv Term course on the study of cristals, emphasizing Inhorntory investigations and a pcrsonalired system ~f instrurtiun u,as e n t h ~ ~ s i a s t i c areceived ll~ hy students at our insrir~~tion. The rourse was intended for undergraduate chemisrrv students who wanted to do interesting and somewhat un&al chemistry during the January Term hut were not especially interested in the widely offered environmental chemistry topics courses. The course emphasized programmed learning materials and relatively inexpensive investigations into several properties of a variety of crystals. The degree of course difficulty was established to he attainable by a good first-year chemistry student and sufficiently flexible to challenge an advanced undergraduate by its rigor. The Appleton-Century-Crofts crystallography programmed course in three dimensions ( I ) provided the formal structure of the course and the directed instruction. Lectures were limited to course organization and the highlighting of a few of the topics studied. For the most part, students devoted morning sessions to the programmed material (with instructor available), and afternoon sessions to the preparation and analysis of crystals. The programmed materials covered a series of topics which included the building of plastic-sphere models to illustrate the discussiou. Topics included "Packing: The Crystal as an Assembly of Spheres," "The Packing of Spheres of Different Sizes-Interstitial Space," "Unit Cells and Space Lattices," "Miller Indices: Representation of Planes and Directions," "The Seven Crystal Systems," "The Fourteen Bravois Lattices," "Introduction to X-Ray Diffraction," "Laue Patterns,'' and "Powder Patterns: The Debye-Scherrer Method."
Figure 2. Ready-Mount slide pdarizer.
224 /
Journal of Chemical Education
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The laboratory investigations entailed essentially three emphases, the major emphasis heing devoted to the growing of crystals and studies of their properties with polarized light. Bell Telephone crystal kits (2) were used for these studies. As an extension of that work, several students hecame fascinated with the observations of crystal growth under polarized light, and succeeded in producinp photographs-depicting the successive growth patterns. A photograph typical of the final growth patterns is shown in Figure 1. Suggested techniques are given below. A variation of the typical crystal growths was undertaken by some advanced students to produce crystals of organic intermolecular complexes (3). Glycine and neutral salts (principally BaC12) were used in varying ratios to form complexes which could he analyzed for densities, transition and melting points, water of hydration, and mole ratios of constituents such as chlorides. Suggested techniques are given below. Additional studies might he undertaken with the glycine-salt crvstals for further characterization. Also. for institutions having X-ray diffraction equipment, an; of the crvstals mieht be studied bv that procedure. keferenci volumes were made readily available to the students, such as those on the growing of crystals (4-61, a few of the classical works in crystallography (7-9), as well as some others. Photographing Crystals Benzoic acid and barium chloride crystals were photo- of -eraohed . throueh crossed oolarizers a t successive staees crystal development. A polarizing slide is made by taping ~ l a s t i coolarizine film in crossed ~ o s i t i o non both halves of H ~ o d ~~e ka d < ~ o u n2 t X 2 siide. The slide mount is hinged with cellophane tape and folds over two microscope slides, trimmed to the size of the Ready-Mount (see Fig. 2). The henzoic acid melt (mp 122°C) is prepared by gently heating several flakes of the acid placed between two standard microscope slides. The slide sandwich is allowed to cool to the touch before heing placed into the hinged Ready-Mount polarizer. Similarly, a concentrated solution of BaC12 is placed between two microscope slides and then placed within the Ready-Mount. Two prockdures were used to take the pictures of the
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Figure 1. Photograph of benmic acid crystal growth.
Figure 3. Slide ing crystals.
