London Dispersion Force Definition

London dispersion force is a weak intermolecular force between two atoms or molecules in close proximity to each other. The force is a quantum force generated by electron repulsion between the electron clouds of two atoms or molecules as they approach each other.

The London dispersion force is the weakest of the van der Waals forces and is the force that causes nonpolar atoms or molecules to condense into liquids or solids as the temperature is lowered.  Even though it is weak, of the three van der Waals forces (orientation, induction, and dispersion), the dispersion forces are usually dominant. The exception is for small, readily polarized molecules, such as water molecules.

The force gets its name because Fritz London first explained how noble gas atoms could be attracted to each other in 1930. His explanation was based on the second-order perturbation theory. London forces (LDF) are also known as dispersion forces, instantaneous dipole forces, or induced dipole forces. London dispersion forces may sometimes be loosely referred to as van der Waals forces.

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Intermolecular Force Definition in Chemistry

By Anne Marie Helmenstine, Ph.D.

Causes of London Dispersion Forces

When you think of electrons around an atom, you probably picture tiny moving dots, spaced equally around the atomic nucleus. However, electrons are always in motion, and sometimes there are more on one side of an atom than on the other. This happens around any atom, but it's more pronounced in compounds because electrons feel the attractive pull of the protons of neighboring atoms. The electrons from two atoms can be arranged so that they produce temporary (instantaneous) electric dipoles. Even though the polarization is temporary, it's enough to affect the way atoms and molecules interact with each other. Through the inductive effect, or -I Effect, a permanent state of polarization occurs.

London Dispersion Force Facts

Dispersion forces occur between all atoms and molecules, regardless of whether they are polar or nonpolar. The forces come into play when the molecules are very close to each other. However, London dispersion forces are generally stronger between easily polarized molecules and weaker between molecules that are not easily polarized.

The magnitude of the force is related to the size of the molecule. Dispersion forces are stronger for larger and heavier atoms and molecules than for smaller and lighter ones. This is because the valence electrons are farther away from the nucleus in large atoms/molecules than in small ones, so they are not as tightly bound to the protons.

The shape or conformation of a molecule affects its polarizability. It's like fitting together blocks or playing Tetris, a video game—first introduced in 1984—that involves matching tiles. Some shapes will naturally line up better than others.

Consequences of London Dispersion Forces

The polarizability affects how easily atoms and molecules form bonds with each other, so it also affects properties such as melting point and boiling point. For example, if you consider Cl₂ (chlorine) and Br2 (bromine), you might expect the two compounds to behave similarly because they are both halogens. Yet, chlorine is a gas at room temperature, while bromine is a liquid. This is because the London dispersion forces between the larger bromine atoms bring them close enough to form a liquid, while the smaller chlorine atoms have enough energy for the molecule to remain gaseous.