Figure 1.

An S₂ molecule (the dark gray disk) surrounded by its “action sphere” relative to the S₁ molecules (the light gray disks). Both species are “solute” molecules that move about by diffusion in a sea of very many much smaller “solvent” molecules (the small dark disks). Of interest are collisions of the S₂ molecule with any S₁ molecule, an event that happens when the center of some S₁ molecule touches the surface of the action sphere.