London Dispersion and Polarisability

Introduction

Even molecules with no permanent dipole attract one another. Their electron distributions fluctuate, creating temporary polarisation that can correlate with a neighbouring particle. London dispersion explains why nonpolar gases can condense and why larger carbon chains often have higher boiling points.

Core explanation

An isolated neutral molecule can have zero average dipole while its instantaneous electron density is uneven. A temporary dipole can induce a related response in a nearby molecule, and correlated fluctuations produce an attractive dispersion interaction. The explanation is not that electrons orbit in simple classical paths; quantum electron fluctuations and polarisation underlie the effect.

All atoms and molecules have dispersion interactions. H₂O, despite hydrogen bonding, still has dispersion. Nonpolar N₂ and Ar rely strongly on dispersion for condensation in the absence of permanent dipoles. Calling a species “nonpolar” only says its time-averaged permanent dipole is zero; it does not mean its electron cloud is rigid.

Polarisability often grows with electron-cloud size and diffuseness. Down a noble-gas group, larger atoms are more polarisable and generally have stronger dispersion and higher boiling points than smaller members. Similar trends can occur in homologous hydrocarbon chains as more atoms and electrons are added. However, molecular shape influences how much surface can approach a neighbour; a long unbranched chain can make broad contact, while a compact branched isomer often has less effective contact.

Compare n-pentane and a highly branched C₅H₁₂ isomer. They have the same molecular formula and total electron count, yet different shapes and packing. The less-branched form commonly has a higher boiling point because its molecules can have more extended contact and stronger overall dispersion in the liquid. This is a trend, not a claim that branching changes the number of electrons.

Dispersion should not be judged only by molar mass across unrelated molecules. Strong permanent dipoles or hydrogen bonds can outweigh a simple mass trend, and branching or rigidity can shift contact. A fair qualitative comparison identifies which features are held constant and which change.

Step-by-step reasoning

1. Identify whether molecules have permanent dipoles; dispersion exists regardless. 2. Compare electron-cloud size and likely polarisability. 3. Compare molecular shapes and contact surface. 4. Relate stronger attractions to phase-change energy cautiously. 5. Note other force types that may also contribute.

Visual explanation

Draw two nonpolar molecules with initially even electron clouds, then shade one side of the first temporarily darker and show induced opposite shading in the neighbour. Beside draw straight and compact C₅H₁₂ isomers making different contact areas.

Real-world analogy

A flexible cushion deforms more under a nearby object than a stiff one. A larger diffuse electron cloud is more polarisable than a compact cloud. The analogy suggests responsiveness but does not describe quantum correlation exactly.

Real-world example

Noble gases are monatomic and have no permanent molecular dipole, yet they can liquefy at low temperatures. Dispersion attractions between their atoms provide the intermolecular cohesion needed for condensed phases.

Why?

Why does a long chain often have stronger dispersion than a short one? It has more electrons and a larger area that can interact with neighbours, increasing the total correlated fluctuation attraction in a suitable packing arrangement.

Common misconception

“Dispersion forces occur only in nonpolar substances.” They occur in every atom and molecule; they are simply the dominant named attraction in many nonpolar samples.

Worked example

Compare methane CH₄ and butane C₄H₁₀ as nonpolar hydrocarbons under the same pressure. Both have dispersion, but butane has more electrons and a larger molecular surface, so its dispersion interactions are generally stronger. It therefore has a higher boiling point. This prediction does not require inventing hydrogen bonding in butane or claiming methane has no attractions.

Quick check

1. Can a molecule with zero permanent dipole have dispersion forces? Answer: Yes. Temporary electron-density fluctuations occur even in nonpolar molecules.

Exam focus

Use size, polarisability and contact area together. State that dispersion coexists with hydrogen bonding and dipole interactions. Avoid saying “larger mass” is the force itself.

Advanced insight

London's quantum treatment relates dispersion to correlated fluctuations in electron distributions. A full interaction also includes repulsion at very short range, so attractive dispersion does not pull molecules into unlimited overlap.

Summary

Dispersion arises from correlated temporary polarisation and operates between all particles. Larger, more polarisable clouds and greater contact often strengthen it, helping explain boiling trends within related nonpolar families.

Practice questions

1. Do Ar atoms attract one another despite lacking permanent dipoles? Answer: Yes, through dispersion. 2. Which typically has stronger dispersion, methane or butane? Answer: Butane, with more electrons and a larger contact surface. 3. Why can branched and straight isomers boil differently? Answer: Their different shapes affect molecular contact and total dispersion interactions. 4. Does water lack dispersion because it hydrogen-bonds? Answer: No. Dispersion is present alongside hydrogen bonding.