AP® Chemistry · Unit 3 · Topic 3.1
London Dispersion Forces
London dispersion forces come from fleeting, random shifts in electron position that create instant dipoles, which induce matching dipoles on neighboring atoms or molecules. Every particle has them, and they grow stronger as electron clouds get bigger and more polarizable.
Electrons are never sitting still or evenly spread. At any given instant, more of them might happen to be on one side of an atom than the other, creating a tiny, temporary dipole. That lopsided charge distribution pushes on the electron cloud of a neighboring atom, squeezing its electrons away and inducing a matching dipole there. The two flicker in sync for a moment, attract each other, then vanish and reform elsewhere. This is a London dispersion force, and it exists between every pair of atoms or molecules, polar or not, because every one of them has electrons in motion.
The size of this effect depends on polarizability: how easily an electron cloud can be distorted. More electrons, and electrons held farther from the nucleus, are easier to push around, so bigger atoms and molecules feel stronger dispersion forces. Shape matters too. Pentane is a straight chain and can lie alongside its neighbors with lots of surface touching; neopentane is a compact, branched blob with much less contact area. More contact means more momentary dipole pairing, which is why pentane boils 27 degrees higher despite having the identical formula and electron count.
This is why dispersion forces are not automatically the weakest force at the table. Dipole-dipole attractions and hydrogen bonds come from permanent charge separation, but they are usually weaker in absolute strength than people assume, and a large, electron-rich nonpolar molecule can out-muscle them through sheer polarizability. That sets up the rest of the unit: once dispersion forces are understood as universal and size-dependent, you can rank real substances by boiling point using the full mix of forces at play, including dipole-dipole and hydrogen bonding layered on top.
Key terms
- London dispersion force
- An attractive force between particles caused by temporary, instantaneous dipoles induced by shifting electron clouds.
- Instantaneous dipole
- A brief, random uneven distribution of electrons that creates a momentary separation of charge.
- Induced dipole
- A temporary dipole created in one particle's electron cloud as a response to a nearby dipole.
- Polarizability
- How easily an electron cloud can be distorted, generally increasing with more electrons and larger atomic or molecular size.
Watch it explained
- How it worksHow does a fleeting lopsided electron cloud on one atom induce a dipole on its neighbour?
- How it worksWhy does pentane boil 27 degrees hotter than its branched twin neopentane?
- Common mistakeWhy isn't a dispersion force always the weakest force in play between two molecules?
- Going furtherWhat happens to the boiling points as you walk down the halogens from F2 to I2?
Check yourself
End of topic.