London dispersion force
These fluctuations create instantaneous electric fields which are felt by other nearby atoms and molecules, which in turn adjust the spatial distribution of their own electrons.
While the detailed theory requires a quantum-mechanical explanation (see quantum mechanical theory of dispersion forces), the effect is frequently described as the formation of instantaneous dipoles that (when separated by vacuum) attract each other.
The magnitude of the London dispersion force is frequently described in terms of a single parameter called the Hamaker constant, typically symbolized
For atoms that are located closer together than the wavelength of light, the interaction is essentially instantaneous and is described in terms of a "non-retarded" Hamaker constant.
[3][4][5] While the London dispersion force between individual atoms and molecules is quite weak and decreases quickly with separation
, in condensed matter (liquids and solids), the effect is cumulative over the volume of materials,[6] or within and between organic molecules, such that London dispersion forces can be quite strong in bulk solid and liquids and decay much more slowly with distance.
The effects of London dispersion forces are most obvious in systems that are very non-polar (e.g., that lack ionic bonds), such as hydrocarbons and highly symmetric molecules like bromine (Br2, a liquid at room temperature) or iodine (I2, a solid at room temperature).
Liquification of oxygen and nitrogen gases into liquid phases is also dominated by attractive London dispersion forces.
Larger and heavier atoms and molecules exhibit stronger dispersion forces than smaller and lighter ones.
[8] This is due to the increased polarizability of molecules with larger, more dispersed electron clouds.
The same increase of dispersive attraction occurs within and between organic molecules in the order RF, RCl, RBr, RI (from smallest to largest) or with other more polarizable heteroatoms.
[9] Fluorine and chlorine are gases at room temperature, bromine is a liquid, and iodine is a solid.
The perturbation is because of the Coulomb interaction between the electrons and nuclei of the two moieties (atoms or molecules).
Thus, no intermolecular antisymmetrization of the electronic states is included, and the Pauli exclusion principle is only partially satisfied.
Additionally, an approximation, named after Albrecht Unsöld, must be introduced in order to obtain a description of London dispersion in terms of polarizability volumes,
The "explanation" of the dispersion force as the interaction between two such dipoles was invented after London arrived at the proper quantum mechanical theory.
The authoritative work[13] contains a criticism of the instantaneous dipole model[14] and a modern and thorough exposition of the theory of intermolecular forces.
Dispersion forces are usually dominant over the three van der Waals forces (orientation, induction, dispersion) between atoms and molecules, with the exception of molecules that are small and highly polar, such as water.
