The strength of association of ionic groups of opposite charge depends on the chemical nature of the ions, the distance between them, and the polarity of the medium. In general, the strength of the interaction between two charged groups (i.e., the energy required to completely separate them in the medium of interest) is less than the energy of a covalent bond but greater than the en- ergy of a hydrogen bond (Table 2-1). The noncovalent associations between neutral molecules, collectively known as van der Waals forces, arise from electrostatic interactions among