Van der Waals force

The van der Waals equation is an equation of state that can be derived from a special form of the potential between a pair of molecules (hard-sphere repulsion and R<sup>-6</sup> van der Waals attraction). In chemistry van der Waals forces are a sometimes used as synonym for non-covalent or intermolecular forces—forces that are weak compared to those appearing in covalent bonding. However, different texts designate different components of the intermolecular potential by the name of the Dutch physicist J. D. van der Waals. To explain this, it must be noted that in general an intermolecular potential has a repulsive part, prohibiting the collapse of molecular complexes, and an attractive part, causing deviations from the ideal gas law and condensation of molecular gases to liquids. The attractive part consists of (electrostatic) interactions between charges (in the case of molecular ions), dipoles (in the case of molecules without inversion center), quadrupoles (all molecules with symmetry lower than cubic), and in general between permanent multipoles. The electrostatic interaction is sometimes called after Keesom. Another source of attraction is induction (aka polarization), which is the interaction between a permanent multipole on one molecule with an induced multipole on another. This interaction is sometimes called after Debye. The final attraction is usually named after London who himself called it dispersion. This is the only attraction experienced by noble gas atoms, but it is operative between any pair of molecules, irrespective of their symmetry. As stated, some texts mean by van der Waals force the totality of forces (including repulsion), others mean all the attractive forces (and then speak of van der Waals-Keesom, van der Waals-Debye, and van der Waals-London), and some texts use van der Waals force solely as a synonym for the London/dispersion force.