Chemical Bonding and Molecular Geometry

Induced Dipoles and London Dispersion Forces

Nonpolar molecules have intermolecular interactions called London dispersion forces. London dispersion forces increase with molecule size.

Nonpolar molecules are affected by nearby charges to differing degrees. The electrons of some molecules can be easily affected. Electrons of other molecules are more resistant. The ability for polarity to change because of the presence of an applied electric field is called polarizability.

Helium gas consists of single atoms. Helium is a small atom, and as a lone atom, it is nonpolar by default. However, helium can be turned into a liquid at extremely low temperatures. All liquids have intermolecular forces, so there must be an intermolecular force acting between helium atoms to allow for condensation to a liquid state.

In 1930 Fritz London, a German American physicist, developed an explanation. London proposed that in a nonpolar atom, such as helium, the average electron distribution is spherical. However, there may be imbalances in the electron distribution at any given moment; both electrons can momentarily be on the same side of the helium atom, for example. Electrons move at relativistic speeds; such imbalances will almost immediately correct themselves. Nevertheless, a nonpolar atom, such as helium, can become a dipole for short periods of time. A dipole that forms because of a short-lived imbalance in charge distribution in an otherwise nonpolar atom or molecule is termed an instantaneous dipole.

Instantaneous dipoles can induce charge distribution changes in other nearby atoms and molecules, creating an induced dipole, which is a polarized bond induced by an electric field. A London dispersion force is an interaction between induced, instantaneous dipoles in nonpolar molecules or atoms. London dispersion forces are proportional to the polarizability of a molecule, which is also proportional to Z (atomic number). London dispersion forces are very weak in small molecules. In large molecules the electron configuration is often prone to changing more dramatically. The magnitude of London dispersion forces increases with molecular size, and in large molecules they become the dominant intermolecular force. Therefore, it is more common to see London dispersion forces in larger atoms than small atoms.

One of the first scientists working on intermolecular forces was Dutch scientist Johannes van der Waals. A dipole-dipole interaction, a London dispersion force, or an intermolecular force between permanent molecular dipoles is known as a van der Waals force.

A nonpolar molecule does not normally carry partial charges. If, however, an ion or a permanent dipole is close to a nonpolar molecule, the electron distribution of the nonpolar molecule is affected. The presence of an ion or a permanent dipole may cause a nonpolar molecule to form a temporary dipole.