91Ó°ÊÓ

(a) The quantum mechanical description of many-electron atoms makes it difficult to define a precise atomic radius because it provides a probabilistic description of the electron's location rather than a fixed path. With multiple electrons exhibiting delocalized behavior, determining a clear boundary for the atom becomes challenging. Hence, an average or most probable distance is generally used to describe the atomic radius. (b) When nonbonded atoms come up against one another, the proximity of the nuclear centers is determined by the balance between attractive forces (such as van der Waals' interactions) and repulsive forces (involving electron cloud repulsion and the Pauli Exclusion Principle). The equilibrium distance between two nuclei is established when the net effect of these forces is balanced.