Intermolecular forces
Forces between molecules are weaker than bonds within them, but they decide whether a substance is a gas, liquid, or solid at room temperature.
Intramolecular versus intermolecular
Covalent bonds hold atoms together within a molecule; intermolecular forces attract separate molecules to each other. Boiling water breaks intermolecular forces, not O–H bonds. The steam is still \(\text{H}_2\text{O}\).
London dispersion forces
All molecules experience dispersion forces: temporary dipoles from moving electrons induce dipoles in neighbours. They grow with molecular size and surface area, because larger electron clouds are more easily distorted. That is why the halogens go from gas (\(\text{F}_2\)) to liquid (\(\text{Br}_2\)) to solid (\(\text{I}_2\)) down the group.
Dipole-dipole forces
Polar molecules attract each other through their permanent dipoles, the positive end of one near the negative end of another. These forces add to dispersion forces, so polar molecules usually boil higher than nonpolar molecules of similar size.
Hydrogen bonding
When hydrogen is bonded to N, O, or F, it carries a strong partial positive charge and is attracted to lone pairs on N, O, or F of neighbouring molecules. Hydrogen bonds are the strongest intermolecular force. They give water its unusually high boiling point and hold the two strands of DNA together.
Consequences of hydrogen bonding in water
Water boils at \(100^\circ\text{C}\), far higher than similar-sized molecules like \(\text{H}_2\text{S}\) (about \(-60^\circ\text{C}\)). Ice is less dense than liquid water because hydrogen bonds lock molecules into an open hexagonal lattice, so ice floats and lakes freeze from the top, protecting aquatic life.
Properties explained by intermolecular forces
Stronger intermolecular forces mean higher boiling and melting points, higher viscosity, higher surface tension, and lower vapour pressure. Water striders walk on water because of surface tension; honey flows slowly because of strong intermolecular attractions.
Worked example
Rank by boiling point: \(\text{CH}_4\), \(\text{HCl}\), \(\text{H}_2\text{O}\). Methane has only dispersion forces (lowest). HCl adds dipole-dipole forces. Water has hydrogen bonding (highest). Order: \(\text{CH}_4 < \text{HCl} < \text{H}_2\text{O}\).
Common mistakes
Thinking boiling breaks covalent bonds. Calling any hydrogen-containing molecule hydrogen-bonded; hydrogen must be bonded to N, O, or F. Forgetting that dispersion forces exist in all molecules and can dominate for large molecules.
Key takeaways
Intermolecular forces act between molecules: dispersion (all molecules, growing with size), dipole-dipole (polar molecules), and hydrogen bonding (H on N, O, F). Stronger forces raise boiling points, viscosity, and surface tension. Hydrogen bonding explains water's remarkable properties and DNA's structure.
Flashcard
Attraction between molecules
Check your understanding
Which substance has hydrogen bonding between its molecules?