It is gratifying to launch the third edition of our book. Its coming to life testi?es about the task it has ful?lled in the service of the com- nity of chemical research and learning. As we noted in the Prefaces to the ?rst and second editions, our book surveys chemistry from the point of view of symmetry. We present many examples from ch- istry as well as from other ?elds to emphasize the unifying nature of the symmetry concept. Our aim has been to provide aesthetic pl- sure in addition to learning experience. In our ?rst Preface we paid tribute to two books in particular from which we learned a great deal; they have in?uenced signi?cantly our approach to the subject matter of our book. They are Weyl’s classic, Symmetry, and Shubnikov and Koptsik’s Symmetry in Science and Art. The structure of our book has not changed. Following the Int- duction (Chapter 1), Chapter 2 presents the simplest symmetries using chemical and non-chemical examples. Molecular geometry is discussed in Chapter 3. The next four chapters present gro- theoretical methods (Chapter 4) and, based on them, discussions of molecular vibrations (Chapter 5), electronic structures (Chapter 6), and chemical reactions (Chapter 7). For the last two chapters we return to a qualitative treatment and introduce space-group sym- tries (Chapter 8), concluding with crystal structures (Chapter 9). For the third edition we have further revised and streamlined our text and renewed the illustrative material.

We have been gratified by the warm reception of our book, by reviewers, colleagues, and students alike. Our interest in the subject matter of this book has not decreased since its first appearance; on the contrary. The first and second editions envelop eight other symmetry-related books in the creation of which we have participated: I. Hargittai (ed.), Symmetry: Unifying Human Understanding, Pergamon Press, New York, 1986. I. Hargittai and B. K. Vainshtein (eds.), Crystal Symmetries. Shubnikov Centennial Papers, Pergamon Press, Oxford, 1988. M. Hargittai and I. Hargittai, Fedezziikf6l a szimmetri6t! (Discover Sym- try, in Hungarian), Tank6nyvkiad6, Budapest, 1989. I. Hargittai (ed.), Symmetry 2: Unifying Human Understanding, Pergamon Press, Oxford, 1989. I. Hargittai (ed.), Quasicrystals, Networks, and Molecules of Fivefold Sym- try, VCH, New York, 1990. I. Hargittai (ed.), Fivefold Symmetry, World Scientific, Singapore, 1992. I. Hargittai and C. A. Pickover (eds.), Spiral Symmetry, World Scientific, Singapore, 1992. I. Hargittai and M. Hargittai, Symmetry: A Unifying Concept, Shelter Publi- tions, Bolinas, California, 1994. We have also pursued our molecular structure research, and some books have appeared related to these activities: vi Preface to the Second Edition I. Hargittai and M. Hargittai (eds.), Stereochemical Applications of Gas-Phase Electron Diffraction, Parts A and B, VCH, New York, 1988. R. Gillespie and I. Hargittai, VSEPR Model of Molecular Geometry, Allyn and Bacon, Boston, 1991. A. Domenicano and I. Hargittai (eds.), Accurate Molecular Structures, Oxford University Press, Oxford, 1992.

This book is written to introduce a new approach to stereochemical problems and to combinatorial enumerations in chemistry. This approach is based on group the ory, but different from conventional ways adopted by most textbooks on chemical group theory. The difference sterns from their starting points: conjugate subgroups and conjugacy classes. The conventional textbooks deal with linear representations and character ta bles of point groups. This fact implies that they lay stress on conjugacy classesj in fact, such group characters are determined for the respective conjugacy classes. This approach is versatile, since conjugacy classes can be easily obtained by ex amining every element of a group. It is unnecessary to know the group-subgroup relationship of the group, which is not always easy to obtain. The same situa tion is true for chemical enumerations, though these are founded on permutation groups. Thus, the P6lya-Redfield theorem (1935 and 1927) uses a cycle index that is composed of terms associated with conjugacy classes.