London dispersion forces are weak attractions between atoms and molecules caused by momentary changes in the distribution of electrons. These changes can create temporary electric charges that attract nearby particles, allowing even electrically neutral atoms and nonpolar molecules to exert forces on one another.
Although an individual London dispersion force is usually weak compared with a chemical bond or some other intermolecular attractions, these forces play an essential role in the physical world. They help explain why gases can become liquids, why certain substances have relatively high boiling points, and why molecules without permanent electric charges can still attract one another.
London dispersion forces occur in all atoms and molecules, making them the most widespread type of intermolecular force. Understanding how they arise helps explain many familiar properties of matter, from the behavior of liquid hydrocarbons to the ability of some animals to cling to smooth surfaces.
What causes London dispersion forces?
Atoms contain positively charged protons in their nuclei and negatively charged electrons surrounding those nuclei. In an electrically neutral atom, the total positive charge balances the total negative charge.
However, the electrons are not stationary. Their distribution changes continuously, and the electron cloud surrounding an atom can become uneven for brief periods. Even when the average distribution is symmetrical, momentary fluctuations can create a temporary dipole.
A dipole is a separation of electric charge, with one region becoming relatively negative and another relatively positive. In a temporary dipole, this separation lasts only briefly before the electron distribution changes again.
When a temporary dipole forms in one atom or molecule, its electric field can influence the electrons in a nearby particle. The neighboring particle’s electron cloud shifts in response, producing an induced dipole. The two particles then attract because their charge distributions are arranged so that opposite charges are closer together.
This attraction is called a London dispersion force.
The process does not require atoms or molecules to carry permanent electric charges. Nor does it require them to have permanent dipoles. The attraction emerges from fluctuations in electron distribution and the way those fluctuations influence nearby particles.
How London dispersion forces work, step by step
The mechanism can be understood by considering two neighboring atoms, each of which is electrically neutral overall.
First, the electrons in one atom momentarily become distributed unevenly. One side of its electron cloud has a slightly greater concentration of negative charge, while the other side becomes relatively positive.
Next, this temporary dipole affects the neighboring atom. The negative region of the first atom repels electrons in the second atom, shifting its electron cloud away. The second atom develops an induced dipole, with its positive region oriented toward the negative region of the first atom.
The resulting arrangement creates an attractive electrostatic interaction between the atoms.
Because the electron clouds continue to fluctuate, these dipoles are constantly changing. The attraction is therefore not a fixed connection between two particles. Instead, it is a consequence of their correlated electron motion and the resulting electric interactions.
The full quantum-mechanical explanation is more subtle than a simple picture of electrons moving from one side of an atom to another. London dispersion forces arise from fluctuations in the quantum states of electrons and the interactions between neighboring particles. The temporary-dipole model is a useful way to visualize the process, even though it is a simplified representation.
Why all atoms and molecules experience dispersion forces
Every atom and molecule contains electrons, and the distribution of those electrons can fluctuate. As a result, London dispersion forces occur regardless of whether a substance is polar or nonpolar.
A polar molecule, such as water, has a permanent dipole because its electric charge is distributed unevenly on average. A nonpolar molecule, such as nitrogen or methane, does not have a permanent molecular dipole. Nevertheless, both types of molecules experience London dispersion forces.
Even individual noble gas atoms, including helium, neon, and argon, attract one another through dispersion forces. These atoms do not normally form conventional chemical bonds with one another, but their mutual attractions become important under suitable conditions.
For example, when a noble gas is cooled sufficiently, its particles lose enough kinetic energy that intermolecular attractions can help hold them together in a liquid. Helium requires exceptionally low temperatures to liquefy under ordinary pressure, while heavier noble gases such as xenon liquefy at much higher temperatures.
These differences illustrate that dispersion forces are universal but vary considerably in strength.
What determines the strength of London dispersion forces?
Several factors influence how strongly two atoms or molecules attract one another through dispersion forces. The most important include the ease with which their electron clouds can be distorted, the number and distribution of their electrons, and the distance between the particles.
Electron cloud polarizability
Polarizability describes how easily an atom’s or molecule’s electron distribution can be distorted by an electric field.
Particles with highly polarizable electron clouds generally experience stronger London dispersion forces. Their electrons can shift more readily, allowing larger temporary and induced dipoles to develop.
Larger atoms often have greater polarizability because they contain more electrons spread over a larger region. Their outer electrons are generally less tightly confined than those in smaller atoms, making their distributions easier to distort.
This helps explain a familiar trend among the noble gases. Helium has a very small, relatively difficult-to-distort electron cloud, whereas xenon has a much larger, more polarizable one. Xenon therefore experiences stronger dispersion attractions between its atoms.
The trend also helps explain why heavier noble gases generally have higher boiling points than lighter ones.
Number of electrons and molecular size
As a broad rule, larger atoms and molecules tend to have stronger dispersion forces because they often contain more electrons and have more polarizable electron clouds.
Consider methane, ethane, propane, and butane, a series of increasingly large hydrocarbons. As the molecules become larger, their dispersion interactions generally become stronger. Under comparable conditions, this trend contributes to the rise in their boiling points.
However, molecular size alone does not determine the strength of attraction. The arrangement of the electrons and the shape of the molecule also matter. A larger molecule is not automatically more strongly attracted to every possible partner under every circumstance.
Molecular shape and contact area
Molecular shape influences how closely particles can approach one another and how much of their electron clouds can interact.
For example, long, relatively straight molecules can often make more extensive contact with neighboring molecules than compact, highly branched molecules of similar composition. Greater contact can increase the overall dispersion attraction between them.
This helps explain why some straight-chain hydrocarbons have higher boiling points than their more highly branched structural isomers, even though the isomers have the same molecular formula and molar mass.
The effect is not simply a matter of surface area in isolation. Molecular geometry, the distribution of polarizability, and the distances between interacting regions all contribute.
Distance between particles
London dispersion forces depend strongly on the separation between particles. At typical intermolecular distances, bringing particles closer can strengthen their attractive interaction. At extremely short distances, however, the overlap of their electron clouds produces strong repulsion.
The balance between attraction and repulsion determines the preferred spacing of particles in many liquids and molecular solids.
For two isolated particles whose leading interaction is London dispersion, the attractive potential energy at sufficiently large separation commonly varies approximately as the inverse sixth power of the distance. In other words, if the separation increases, the attraction weakens rapidly.
In a simple model, the attractive energy can be expressed as proportional to −1/r⁶, where r is the distance between the particles. This relationship applies in the appropriate long-distance regime; it is not a complete description of the interaction at every separation or in every material.
London dispersion forces compared with other intermolecular forces
London dispersion forces belong to a broader group of attractions called intermolecular forces. These are interactions between separate atoms or molecules, rather than the chemical bonds that hold atoms together within a molecule.
Three commonly discussed types of intermolecular attraction are London dispersion forces, dipole–dipole interactions, and hydrogen bonding.
| Type of interaction | How it arises | Where it occurs |
|---|---|---|
| London dispersion forces | Fluctuating electron distributions create correlated temporary and induced dipoles | All atoms and molecules |
| Dipole–dipole interactions | Permanent molecular dipoles attract one another | Polar molecules |
| Hydrogen bonding | A particularly strong, directional interaction involving hydrogen bonded to a highly electronegative atom and an appropriate neighboring atom | Certain molecules, including water and ammonia |
Dipole–dipole interactions depend on permanent molecular polarity, while hydrogen bonding is a particularly important type of intermolecular interaction with specific structural requirements. London dispersion forces do not require either permanent polarity or hydrogen bonding.
These categories can operate simultaneously. Water molecules, for example, experience hydrogen bonding, dipole–dipole interactions, and London dispersion forces. It would therefore be misleading to describe water’s intermolecular attractions as arising from only one type of force.
The relative importance of each interaction depends on the substances involved and the conditions under which they interact.
Dispersion forces are often weaker than individual hydrogen bonds or favorable permanent dipole–dipole interactions. However, they can become substantial when many atoms and molecules interact over a large contact area. In some systems, dispersion forces are the dominant intermolecular attraction.
Why London dispersion forces affect boiling and melting points
Boiling and melting involve changes in how particles are arranged and move, not simply the breaking of chemical bonds.
In a liquid, particles remain close together but can move past one another. During boiling, molecules separate sufficiently to enter the gas phase. During melting, particles in a solid gain enough freedom of movement to form a liquid.
Intermolecular attractions oppose the separation or rearrangement of particles. When London dispersion forces are stronger, more energy may be needed to separate molecules into a gas, all else being comparable. This often leads to higher boiling points.
The same attractions can influence melting points, although melting behavior is more complicated because it also depends strongly on how efficiently particles pack into a crystal and how stable that crystal structure is.
A comparison of noble gases demonstrates the connection between dispersion forces and boiling points. Their atoms have no permanent molecular dipoles, so dispersion forces are central to their attractions. As the atoms become larger and more polarizable, their dispersion attractions generally strengthen, and their boiling points rise.
The same principle applies to many nonpolar molecular substances. Larger hydrocarbons tend to have higher boiling points than smaller ones because their intermolecular dispersion attractions generally increase with molecular size.
These trends are useful guidelines rather than universal rules. Molecular shape, other intermolecular forces, pressure, and the structure of the condensed phase can all affect the observed transition temperatures.
How dispersion forces allow nonpolar substances to condense
A common misconception is that nonpolar molecules cannot attract one another because they lack permanent dipoles. London dispersion forces show why this is incorrect.
A nonpolar molecule can have an uneven electron distribution at a particular instant even when its average distribution is symmetrical. Its temporary electric field can influence neighboring molecules, creating attractive interactions.
When a nonpolar gas is cooled, the average kinetic energy of its molecules decreases. Eventually, under suitable pressure and temperature conditions, the attractive interactions can help stabilize a liquid state.
This is why substances such as oxygen, nitrogen, methane, and many other nonpolar molecules can exist as liquids. Their particles do not need permanent electric charges to attract one another.
Dispersion forces also help explain why many nonpolar molecular substances can form solids. At sufficiently low temperatures, intermolecular attractions and favorable packing can stabilize an ordered arrangement of particles.
The resulting material is held together by a balance of interactions, including attraction between particles and short-range repulsion between their electron clouds.
The role of London dispersion forces in everyday materials
London dispersion forces influence many properties of materials that people encounter in daily life.
In liquids such as gasoline and other hydrocarbon mixtures, dispersion forces are major contributors to the attraction between molecules. These interactions affect volatility, boiling behavior, and how readily the liquid evaporates.
They also contribute to the behavior of polymers, which are large molecules built from repeating chemical units. In many polymers, attractions between neighboring chains help determine cohesion, flexibility, and mechanical properties. Dispersion forces are especially important in nonpolar polymer materials, although the properties of any particular polymer also depend on its chemical structure and other interactions.
Dispersion forces contribute to the attraction between surfaces, including surfaces made from materials that have no permanent electrical charge. When two surfaces approach closely, the cumulative interactions between many atoms can produce measurable adhesion.
This effect is relevant to the way some small animals and insects grip surfaces. In geckos, for example, microscopic structures on the feet create extensive close contact with a surface, allowing weak intermolecular attractions, including dispersion forces, to add up to substantial adhesion. The effectiveness of this mechanism depends on the detailed geometry of the foot structures and the nature of the surface.
These examples demonstrate an important principle: a weak interaction at the scale of two individual particles can have a significant effect when many such interactions act together.
Why dispersion forces matter in chemistry
London dispersion forces help chemists predict and explain the physical behavior of substances whose properties cannot be understood from chemical bonds alone.
Two molecules can have the same molecular formula but different shapes, leading to differences in how closely they pack and how extensively they interact. Two substances with similar chemical structures can also have different boiling points because their sizes and electron distributions differ.
Dispersion forces are particularly important when comparing nonpolar substances, but they must also be considered in polar molecules, biological structures, and complex materials. Their contributions can influence molecular recognition, the stability of folded structures, and the way molecules associate with one another.
At the atomic scale, these forces arise from quantum-mechanical electron fluctuations. At larger scales, their combined effects influence measurable properties such as boiling points, adhesion, and the cohesion of materials.
London dispersion forces are therefore not merely a minor detail of chemistry. They are a fundamental part of how neutral particles interact, helping explain why matter can condense, why molecules associate, and why the physical properties of substances depend on more than the chemical bonds within them.
