A chemist contemplates models of intermolecular collisions: A game of billiards or a demolition derby

The kinetic molecular theory is one of the major triumphs of the physical sciences. In the following essay, we trace its current quantitative formulation, which was derived from concepts introduced during the early decades of the 19th century. Its evolution incorporated a sequence of refinements of models and of detailed mathematical analyses. Remarkably, it bridges measurable macroscopic properties and molecular parameters. However, for chemical kinetics applications it is incomplete, and its intrinsic conceptual simplifications should not be overlooked when using the theoretically derived relations. Consideration should be given to limits of applicability, and to be cognizant of features that are not fully compatible with other molecular models that are derived from diverse sources. In some measure, quantum mechanical formulations of molecular encounters bypass some of the inadequacies of the classical treatment. Herein, the designation “molecular models” refers not only to the postulated shapes, sizes, and interaction potentials of the elementary particles but also to the underlying mathematical analyses that lead to relations between observable quantities. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 37: 191–200, 2005