Chemistry
London Dispersion Forces
Quick fact
London dispersion forces are the weakest of all intermolecular forces, but they are responsible for the existence of all condensed states of matter—without them, nothing would ever condense into a liquid or solid.
Why this is interesting
Ever wondered why helium, the lightest gas, turns into a liquid at just a few degrees above absolute zero? Or why even nonpolar molecules cling together? It's all due to an incredibly weak—yet universal—force that arises from nothing more than the jitter of electrons.
Read the full explanation
Understanding London Dispersion Forces
Imagine a beach ball (the electron cloud) surrounding a nucleus. Electrons are in constant motion, and at any instant, they might crowd to one side, making that side temporarily more negative and the other side more positive. This creates an instantaneous, fleeting dipole. This dipole can 'push' electrons in a neighboring atom or molecule, creating an induced dipole. The temporary attraction between these fleeting dipoles is the London dispersion force. Even though each individual force is incredibly weak, they occur constantly and add up, making them significant—especially for large molecules.
A deeper explanation
London dispersion forces arise from quantum mechanical fluctuations in the electron distribution of atoms and molecules. At any given moment, the electron cloud may be asymmetric, creating a temporary dipole. This temporary dipole induces a complementary dipole in a neighboring particle, resulting in an attraction. The strength of this force depends on the polarizability of the electron cloud: larger atoms and molecules have more diffuse electron clouds, making them easier to distort and thus experiencing stronger dispersion forces. This explains trends in boiling points—for example, the noble gases boil at increasing temperatures as you go down the periodic table (He to Rn) because their polarizability increases. London dispersion forces are universal, present in all matter, and play a critical role in determining physical properties such as melting and boiling points, even for nonpolar species like hydrocarbons. They are also key in biological systems, such as the stacking of DNA bases, where multiple weak interactions collectively stabilize the structure.