We show that several important concepts of descriptive chemistry, such as
atomic shells, bonding electron pairs and lone electron pairs, may be described
in terms of {\it quantum stress focusing}, i.e. the spontaneous formation of
high-pressure regions in an electron gas. This description subsumes previous
mathematical constructions, such as the Laplacian of the density and the
electron localization function, and provides a new tool for visualizing
chemical structure. We also show that the full stress tensor, defined as the
derivative of the energy with respect to a local deformation, can be easily
calculated from density functional theory.Comment: 5 pages, 2 figure