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CHEM 121 Studioby Learn4Less · UBC CHEM 121

CHEM 121 glossary

London dispersion forces

The attraction between an instantaneous dipole in one molecule and the dipole it induces in a neighbour. They act between all molecules and atoms, and grow stronger with more electrons (greater polarizability) and more contact area.

London dispersion forcesLeft: at one instant the electron cloud of an atom is lopsided, giving a temporary dipole. Right: that dipole pushes the electrons of the neighbouring atom away, inducing a dipole in it. The two temporary dipoles attract.δ+δ−δ+δ−instantaneous dipoleinduced dipoleact between all molecules and atomsbigger electron cloud → more polarizable → stronger

A momentary dipole in one atom induces a matching dipole next door.

Example

Methane and octane are both nonpolar alkanes with only dispersion forces, yet CHX4\ce{CH4} boils at 111.7 K and CX8HX18\ce{C8H18} at 398.8 K: octane has many more electrons and a much longer chain.

See alsoPolarizabilityIntermolecular forcesBoiling point