What Are London Dispersion Forces?
London dispersion forces are the weakest intermolecular forces, arising from temporary shifts in electron clouds that create fleeting attractions between molecules. They're found in all molecules and are the only force between nonpolar substances.
London dispersion forces result from temporary dipoles when electrons briefly accumulate on one side of a molecule. All molecules experience them; they're especially important in noble gases and nonpolar hydrocarbons.
- 1↓Random electron shiftElectrons momentarily gather on one side of the molecule
- 2↓Temporary dipole formsThe molecule becomes slightly polar with δ+ and δ- ends
- 3↓Induces neighboring dipoleThe negative end attracts electrons in the next molecule
- 4Weak attraction resultsTiny but measurable force pulls molecules together
Step-by-step worked examples
Why do nonpolar molecules like N₂ condense at very low temperatures?
N₂ is nonpolar; only London forces hold it. At room temp, kinetic energy > London forces → gas. At −196 °C, kinetic energy < London forces → liquid.
Which is stronger: London forces between Cl₂ or Br₂?
Br₂ has more electrons (35 vs 17) → larger electron cloud. Larger cloud → stronger temporary dipoles. Br₂ experiences stronger London forces.
Why does hexane (C₆H₁₄) boil at 69 °C while methane (CH₄) boils at −161 °C?
Hexane is longer (24 electrons) → larger surface area for forces. Methane is tiny (10 electrons). More electrons = stronger London forces → higher boiling point.
Flashcards
Quick quiz
Q1.London dispersion forces arise from…
Q2.Which substance experiences ONLY London forces?
Q3.More electrons = …
Q4.Boiling point trend in the halogen group (F₂, Cl₂, Br₂, I₂)?
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Common mistakes
London forces only exist in nonpolar molecules. — Correct: They exist in ALL molecules; just harder to detect with other forces present.
London forces are as strong as hydrogen bonds. — Correct: London forces are the weakest intermolecular force.
London forces are permanent dipole–dipole forces. — Correct: They arise from TEMPORARY, transient dipoles.
Larger atoms always have stronger London forces. — Correct: Stronger forces depend on polarizability and electron count, not just size.
FAQ
What are London dispersion forces?
Weak intermolecular attractions from temporary electron density shifts creating fleeting dipoles.
Why do all molecules have London forces?
All molecules have electron clouds that can undergo temporary polarization.
How do London forces compare to hydrogen bonds?
London forces are much weaker (0.05–40 kJ/mol) than hydrogen bonds (10–40 kJ/mol).
Why does boiling point increase down a halogen group?
Atomic radius and electron count increase, allowing stronger temporary dipoles.