Van der Waals forces
Summary
Van der Waals forces are weak attractions between molecules or nonbonded atoms. Broadly they include London dispersion and dipole-dipole forces; some texts mean dispersion alone. Testable point: dispersion forces strengthen with size and surface area, so boiling point generally rises.
Detail
Meaning: Van der Waals forces are weak, short-range attractions between molecules or nonbonded atoms, with no electrons shared or transferred. The broad usage includes London dispersion, dipole-dipole and dipole-induced dipole forces; the narrow usage means London dispersion alone. Dispersion arises when a fluctuating electron cloud makes a momentary dipole that induces a dipole in a neighbor. All molecules show it, polar or not, and it grows with more electrons, higher polarizability and greater surface contact.
On the MCAT: Expect rankings of boiling point, melting point or vapor pressure: stronger forces mean higher boiling point and lower vapor pressure. Common trends are noble gases, halogens (F₂ is a gas, I₂ a solid) and straight-chain alkanes rising with size, and a straight chain boiling higher than its branched isomer. Real-gas questions use the van der Waals equation, whose terms correct for attractions and molecular volume.
Common confusion: Nonpolar does not mean no attraction; nonpolar substances condense because of dispersion. Boiling overcomes intermolecular forces, not covalent bonds. For similar-sized molecules hydrogen bonding is stronger, but a much larger molecule can have more total dispersion and may boil higher.
Sources
- Chemistry: The Central Science
- General Chemistry: Principles and Modern Applications
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.