Nature of the interaction
London dispersion forces are intermolecular attractions that arise from momentary fluctuations in electron distribution. At any instant, the electrons in an atom or molecule may be distributed unevenly, producing a temporary dipole. That temporary dipole can distort the electron cloud of a nearby particle and induce a second dipole. The attraction between these oppositely oriented partial charges is called a London dispersion force.
London dispersion forces are present in all atoms and molecules. They are the only intermolecular forces available to nonpolar substances, but they also contribute to the total attraction in polar substances.
Origin in electron motion
The force does not require a permanent dipole. Its origin lies in the constant motion of electrons. A temporary imbalance produces a region of slightly greater electron density, which may be represented as \( \delta^- \), and an opposite region of reduced electron density, represented as \( \delta^+ \). When a neighboring particle responds by polarization, an attractive interaction results.
The interaction is individually weak, but a large number of such attractions can produce significant effects on physical properties such as boiling point, melting behavior, and vapour pressure.
Accurate visual model
Factors that strengthen dispersion forces
London dispersion forces become stronger when particles contain more electrons and when their electron clouds are more polarizable. A larger electron cloud is held less rigidly and can be distorted more easily. Molecular shape also matters. Long, extended molecules often experience stronger dispersion forces than compact molecules of similar molar mass because a larger surface area allows more effective contact between neighboring particles.
Consequences for physical properties
Stronger London dispersion forces usually correspond to higher boiling points and lower vapour pressures. More energy is required to separate particles when intermolecular attraction is stronger. This explains why heavier noble gases have higher boiling points than lighter noble gases and why large nonpolar molecules can exist as liquids or solids even though they do not possess permanent dipoles.
Comparison with other intermolecular forces
| Interaction type | Origin | Present in nonpolar particles? | Typical relative strength |
|---|---|---|---|
| London dispersion forces | Instantaneous dipole-induced dipole attraction | Yes | Weak individually, but can become important in large particles |
| Dipole-dipole forces | Attraction between permanent dipoles | No | Often stronger than dispersion for small polar molecules |
| Hydrogen bonding | Strong dipole interaction involving H bonded to N, O, or F | No | Usually stronger than ordinary dipole-dipole attraction |
Common points of confusion
London dispersion forces are sometimes described as if they occur only in nonpolar molecules. That statement is incomplete. They are universal and operate in every atom and molecule. Nonpolar substances simply rely on them as the dominant intermolecular attraction. Another common mistake is to treat them as negligible. In large molecules, especially those with many electrons and substantial surface area, they can dominate observable physical behavior.
Concise statement
London dispersion forces are temporary dipole-induced dipole attractions that arise from fluctuating electron density, occur in all particles, and become stronger as polarizability and contact surface increase.