London Dispersion Forces

Electrons move constantly, so at any instant one side of an atom is slightly electron rich. That momentary dipole polarises a neighbouring cloud, creating an induced dipole, and the two attract. The attraction is weak but universal.

An instantaneous dipole is a momentary uneven electron distribution, not a permanent positive ion at one end and negative ion at the other. A neighboring cloud responds: electron density is displaced so the adjacent ends tend to have opposite partial charges. The correlated fluctuations lower the pair's energy and produce attraction even though the time-averaged isolated particle is nonpolar. Dispersion is also present in polar molecules and in individual noble-gas atoms; a permanent dipole is unnecessary.

A cloud that distorts easily makes bigger instantaneous dipoles. More electrons and a larger, more diffuse cloud means higher polarisability, so dispersion strength rises down a group and with molar mass within a family.

Polarizability is how easily a cloud is distorted. Large, diffuse clouds are often more polarizable than compact clouds, so electron count and molar mass are useful clues within a related family. Molar mass itself is not an attractive force: nuclei do not attract their neighbors more strongly merely because they weigh more. The explanation should name the cloud and its distortion. For noble gases, going from Ne to Ar to Kr to Xe adds more easily distorted electrons, increasing dispersion and generally increasing boiling point.

n-pentane and neopentane share the formula C₅H₁₂, but the straight chain packs alongside its neighbours over a long surface while the compact sphere touches less. n-pentane boils at 36 °C, neopentane at 10.

The shape comparison is clearest for isomers with the same formula and similar overall polarizability. A long chain can make more extended close contacts with neighboring chains than a compact, highly branched isomer. That often raises its boiling point. This contact explanation is a model for intermolecular cohesion in a liquid, not a claim that molecules are static rigid blocks. It is less reliable for melting points because crystal symmetry and packing can make a compact isomer melt higher despite weaker liquid-phase cohesion.

Use three steps: establish the relevant force, identify the cloud or contact difference, and connect that difference to the property. For Br₂ and I₂, both are nonpolar diatomic molecules and have no permanent dipole or hydrogen-bond donor. I₂ has a larger, more polarizable cloud, so its intermolecular dispersion is stronger. More energy is required to separate its molecules, consistent with a higher boiling point.

Do not say the I–I covalent bond is broken when iodine boils. Intact I₂ molecules enter the vapor. Also avoid comparing two substances solely by the label of their strongest named force. The combined dispersion in a very large molecule can outweigh a hydrogen-bond contribution in a much smaller molecule; use size, shape and measured data when the factors compete. At a fixed temperature, stronger cohesion generally lowers vapor pressure because fewer molecules populate the vapor at equilibrium.