London’s Dispersion Forces London.s dispersive forces
now known merely as dispersive forces
are not due to permanent dipole or induced permanent dipole interactions, or due to interactions between permanent charges on the molecules as in ionic interactions, but are due to transient, random charges, spontaneously generated continuously all over the molecule
. The exact nature of dispersion forces is difficult to describe but the following definition by Glasstone may be helpful:- “-although the physical significance probably can not be clearly defined, it may be imagined that an instantaneous picture of a molecule would show various arrangements of nuclei and electrons having dipole moments. These rapidly varying dipoles when averaged over a large number of configurations would give a resultant of zero. However, at any instant they would offer electrical interactions with another molecule exhibiting the similar dipole configurations, thus, resulting in interactive forces.” Dispersive forces
are the only type of molecular force that can exist to the exclusion of all others (e.g., interaction between one hydrocarbon molecule and another is purely dispersive). All other types of molecular interaction (polar, induced polar, ionic or combinations of such) will be accompanied by dispersive interactions.