London dispersion forces — Science definition
Weak attractions present in ALL molecules, caused by fleeting, temporary dipoles from shifting electrons; stronger in larger molecules with more electrons.
Where it is taught
- Intermolecular Forces — States of Matter & Gases, Physical Science: Chemistry, Grade 11
In use
A molecular interaction present even without permanent polarity is London dispersion forces, and here is their surprising feature — all molecular species can exhibit dispersion, including nonpolar molecules without a permanent net dipole.
Intermolecular Forces
Questions using london dispersion forces
Problem 1
The noble gases Ne, Kr, and Xe have only London dispersion forces. Rank them by increasing boiling point.
- Ne < Kr < Xe
- Xe < Kr < Ne
- Kr < Ne < Xe
- they all boil at the same temperature
Answer and explanation
Ne < Kr < Xe
Dispersion forces increase with size and number of electrons (Ne < Kr < Xe), so boiling point rises in the same order.
Problem 2
Which molecule has ONLY London dispersion forces?
- HF
- H₂O
- NH₃
- CH₄
Answer and explanation
CH₄
CH₄ is nonpolar and cannot hydrogen-bond, so its only intermolecular force is London dispersion; the others can hydrogen-bond.