Symposium on Teaching C~ystallography
How To Use Crystallographic Information in Teaching First-Year Chemistry D. J. M. Bevan and M. R. Taylor The Flinders University of South Australia, Bedford Park 5042. Australia M. Rossi Vassar College, Poughkeepsle, NY 12601 Crystal structures per se are rarely described in any detail in tvnical first-vear chemistw texts, except perhaps for a b r i c a n d usuaily unsatisfactory account of such simple structures as those of rock salt (NaCl), fluorite (CaFd, CsCI, diamond, and graphite. Surh presentations are sumetimes associated with an elementary account of X-ray diffraction, Bragg's law, and perhaps some indication of the procedures by which structures are determined-all this in just one chapter on "the solidstate". Yet throughout such texts there is constant reference to molecular geometries, bonds of various types, bond lengths, and bond strengths, without any indication of the experimental methods by which these data have been derived, among which X-ray crystallography ranks highly. In this article we emphasize the role that crystallography plays and has played in building up a huge body of chemical fact, and by example we attempt toshow howthe teaching of much of this material is illuminated by reference to crystal structures and crystallochemical relationships in all areas of chemistry. Everyone knows that the benzene ring is flat, and that all the carbon-carbon bond lengths are equal, but how was this first confirmed? The structure of the benzene ring was described in a classic paper by Kathleen ~onsdale-(I),who studied hexamethvlbenzene and hexachlorobenzene. Its resemblance to the haphitic structure of carbon, rather than diamond, is also noted in this paper. More recently, the structure of benzene itself has been studied by neutron diffraction (Z), so that more accurate hydrogen atom positions could be found; this resulted in more accurate carboncarbon.bond lengths also (1.398(8) A). By contrast, the cyclohexane ring has the well-known chair form (3),and i t is interesting to note that this same chair form of C-C linkage to give six-membered rings is present in the structure of diamond, but now all such rings share edees in three dimensions. The structure can be described as two interpenetrating cubic close-packed arrays of C atoms. Alternativelv. .. the structure mav be thought of as CCatetrahedra,allin the same orientationand sharkg every
corner to fill snace (Fie. 1). Si has the same structure as diamond, and (.he common abrasive, SIC, likewise. This is also the structure of zinc blende (ZnS). There is, however, another modification of ZnS, wurzite, in which the Zn and S atoms form two interpenetrating hexagonal close-packed arrays. An alternative description iTs that of ZoS4 (or SZnr for that matter) sharing every corner, but now these tetrahedra are no longer all in
