London Dispersion Forces

The London dispersion force is the weakest intermolecular force. The London dispersion force is a temporary attractive force that results when the electrons in two adjacent atoms occupy positions that make the atoms form temporary dipoles. This force is sometimes called an induced dipole-induced dipole attraction. London forces are the attractive forces that cause nonpolar substances to condense to liquids and to freeze into solids when the temperature is lowered sufficiently.

Because of the constant motion of the electrons, an atom or molecule can develop a temporary (instantaneous) dipole when its electrons are distributed unsymmetrically about the nucleus.

A second atom or molecule, in turn, can be distorted by the appearance of the dipole in the first atom or molecule (because electrons repel one another) which leads to an electrostatic attraction between the two atoms or molecules.

Dispersion forces are present between any two molecules (even polar molecules) when they are almost touching.

Molecular Size

Dispersion forces are present between all molecules, whether they are polar or nonpolar.

London dispersion forces tend to be:

Molecular Shape

The shapes of molecules also affect the magnitudes of dispersion forces between them.

= C

= H

n-Pentane is a liquid at 25oC.
Neopentane is a gas at 25oC.

Physical Consequences of London Dispersion Forces

Cl2 and Br2 have approximately the same shape and neither is polar. *Click here to check your answer.