London dispersion force

London dispersion forces (LDF, also known as dispersion forces, London forces, instantaneous dipole–induced dipole forces, or loosely van der Waals forces) are a type of force acting between atoms and molecules. They are part of the van der Waals forces. The LDF is named after the German-American physicist Fritz London. London dispersion forces (LDF, also known as dispersion forces, London forces, instantaneous dipole–induced dipole forces, or loosely van der Waals forces) are a type of force acting between atoms and molecules. They are part of the van der Waals forces. The LDF is named after the German-American physicist Fritz London. The electron distribution around an atom or molecule undergoes fluctuations in time. 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. The net effect is that the fluctuations in electron positions in one atom induce a corresponding redistribution of electrons in other atoms, such that the electron motions become correlated. 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 A. 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. For entities that are farther apart, the finite time required for the fluctuation at one atom to be felt at a second atom ('retardation') requires use of a 'retarded' Hamaker constant. While the London dispersion force between individual atoms and molecules is quite weak and decreases quickly with separation (R) like 1 R 6 {displaystyle {frac {1}{R^{6}}}} , in condensed matter (liquids and solids), the effect is cumulative over the volume of materials,or within and between organic molecules, such that London dispersion forces can be quite strong in bulk solid and liquids and decays much more slowly with distance. For example, the total force per unit area between two bulk solids decreases like 1 R 3 {displaystyle {frac {1}{R^{3}}}} where R is the separation between them. 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 such as bromine (Br2, a liquid at room temperature), iodine (I2, a solid at room temperature). In hydrocarbons and waxes the dispersion forces are sufficient to cause condensation from the gas phase into the liquid or solid phase. Sublimation heats of e.g. hydrocarbon crystals reflect the dispersion interaction. Liquification of oxygen and nitrogen gases into liquid phases is also dominated by attractive London dispersion forces.