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

• Larger and heavier atoms and molecules exhibit stronger dispersion forces than smaller and lighter ones.
• In a larger atom or molecule, the valence electrons are, on average, farther from the nuclei than in a smaller atom or molecule. They are less tightly held and can more easily form temporary dipoles.
• The ease with which the electron distribution around an atom or molecule can be distorted is called the polarizability.

London dispersion forces tend to be:

• stronger between molecules that are easily polarized.
• weaker between molecules that are not easily polarized.

Molecular Shape

• At room temperature, neopentane (C₅H₁₂) is a gas whereas n-pentane (C₅H₁₂) is a liquid.
• London dispersion forces between n-pentane molecules are stronger than those between neopentane molecules even though both molecules are nonpolar and have the same molecular weight.
• The somewhat cylindrical shape of n-pentane molecules allows them to come in contact with each other more effectively than the somewhat spherical neopentane molecules.

		= C = H
n-pentane n-Pentane is a liquid at 25oC.	neopentane Neopentane is a gas at 25oC.

Physical Consequences of London Dispersion Forces

1. Upon cooling, both Cl₂ and Br₂ form solids. Why?
2. At 25^oC, chlorine (Cl₂) is a gas whereas bromine (Br₂) is a liquid. Why?