Weak attractions from temporary dipoles in all molecules, increasing with molar mass and surface area.
Practise Bonding & Molecular Structure All Bonding & Molecular Structure topics
London dispersion forces arise from instantaneous fluctuations in electron distribution, which create temporary dipoles. A temporary dipole induces a dipole in neighbouring molecules, and the resulting attraction is weak and short-ranged. Dispersion forces exist between all atoms and molecules and are the only attraction between nonpolar molecules. Their strength increases with the number of electrons, the polarizability of the species and the surface contact between molecules. Large, elongated molecules therefore have stronger dispersion forces than small compact ones.
Key ideas: Compare by size: more electrons gives more polarizable clouds and higher boiling points. Halogens: F2 gas, Cl2 gas, Br2 liquid, I2 solid at room temperature. Alkanes: boiling point increases by about 20 to 30 degrees per added carbon. Energy falls with distance to the sixth power, so contact surface area is very important.
Common mistakes and connections: A typical mistake is saying nonpolar molecules have no attractions, or that dispersion forces are only found in nonpolar molecules (they exist in all). Another error is ignoring shape: linear n-pentane boils higher than branched neopentane at equal mass. Links to polarizability, boiling points, noble gases, gecko adhesion and the van der Waals radius.
Noble gas boiling points rise with the number of electrons, since larger clouds make stronger instantaneous dipoles and dispersion attraction. The interactive lesson shows this as a live animation or chart.
Boiling points of the noble gases rise steadily from helium at 4 K to xenon at 165 K as the atoms become larger.
Weak attractions from temporary dipoles in all molecules, increasing with molar mass and surface area.
London Dispersion Forces is a beginner topic in Bonding & Molecular Structure. It needs no earlier chemistry lessons and is a good starting point.
Study these earlier topics in Bonding & Molecular Structure first: Benzene Bonding, Aromaticity and Huckel's Rule, Intermolecular Forces.
The next topics in Bonding & Molecular Structure are Dipole-Dipole Forces and Hydrogen Bonding.
Yes. The interactive lesson includes a self-check question on London Dispersion Forces, and the Bonding & Molecular Structure quiz draws on every topic in this area.
London Dispersion Forces is topic 71 of 104 in Bonding & Molecular Structure.