What Are London Dispersion Forces?

London dispersion forces, named after German-American physicist Fritz London, are one of the three Van der Waals intermolecular forces holding molecules together. They are the weakest of the intermolecular forces but strengthen as the atoms at the source of the forces increase in size. While the other Van der Waals forces depend on electrostatic attraction involving polar-charged molecules, the London dispersion forces are present even in materials made up of neutral molecules.

The three intermolecular forces first described by Dutch physicist Johannes Diderik Van der Waals are dipole-dipole forces, dipole-induced dipole forces and London dispersion forces. Dipole-dipole forces involving a hydrogen atom in the molecule are exceptionally strong, and the resulting bonds are called hydrogen bonds. Van der Waals forces help give materials their physical characteristics by influencing how molecules of a material interact and how strongly they are held together.

Intermolecular bonds involving dipole forces are all based on electrostatic attraction between charged molecules. Dipole molecules have a positive and a negative charge at opposite ends of the molecule. The positive end of one molecule can attract the negative end of another molecule to form a dipole-dipole bond.

When neutral molecules are present in the material in addition to dipole molecules, the charges of the dipole molecules induce a charge in the neutral molecules. For example, if the negatively charged end of a dipole molecule comes close to a neutral molecule, the negative charge repels the electrons, forcing them to gather on the far side of the neutral molecule. As a result, the side of the neutral molecule close to the dipole develops a positive charge and is attracted to the dipole. The resulting bonds are called dipole-induced dipole bonds.

London dispersion forces don't require a polar dipole molecule to be present and act in all materials, but they are usually exceedingly weak. The force is stronger for larger and heavier atoms with many electrons than for small atoms, and it can contribute to the physical characteristics of the material.

The London dispersion force is defined as a weak attractive force due to the temporary formation of dipoles in two adjacent neutral molecules. The resulting intermolecular bonds are also temporary, but they form and disappear continuously, resulting in an overall bonding effect.

The temporary dipoles are formed when the electrons of a neutral molecule by chance gather on one side of the molecule. The molecule is now a temporary dipole and can either induce another temporary dipole in an adjacent molecule or be attracted to another molecule that has formed a temporary dipole on its own.

When molecules are large with many electrons, the likelihood that the electrons form an uneven distribution increases. The electrons are farther away from the nucleus and are loosely held. They are more likely to gather on one side of the molecule temporarily, and when a temporary dipole forms, the electrons of adjacent molecules are more likely to form an induced dipole.

In materials with dipole molecules, the other Van der Waals forces dominate, but for materials made up completely of neutral molecules, London dispersion forces are the only active intermolecular forces. Examples of materials made up of neutral molecules include the noble gases such as neon, argon and xenon. London dispersion forces are responsible for the gases condensing into liquids because no other forces hold the gas molecules together. The lightest noble gases, such as helium and neon, have extremely low boiling points because the London dispersion forces are weak. Large, heavy atoms such as xenon have a higher boiling point because the London dispersive forces are stronger for large atoms, and they pull the atoms together to form a liquid at a higher temperature. Although usually comparatively weak, the London dispersion forces can make a difference in the physical behavior of such materials.