Intermolecular Forces

You’ll be able to

Distinguish intermolecular forces from the covalent bonds within a molecule
Rank London dispersion, dipole–dipole, hydrogen bonding, and ion–dipole by strength
Predict relative IMF strength from polarity, polarizability, and molecular size

Forces between molecules, not within them

A bond holds atoms together inside a molecule; an intermolecular force (IMF) is the much weaker attraction between separate molecules. Breaking the O–H bonds in water takes hundreds of kilojoules per mole, but merely pulling water molecules apart to boil it takes far less. IMFs never break during a phase change of a molecular substance — you overcome them, and the molecules stay intact.

The IMF hierarchy, weakest to strongest

Every molecule has London dispersion forces (LDFs) — fleeting attractions from momentary, induced dipoles in the electron cloud. Polar molecules add dipole–dipole attraction between permanent + and − ends. When H is bonded to N, O, or F, an especially strong dipole–dipole called a hydrogen bond appears. Strongest of all in this unit is the ion–dipole force, between a full ionic charge and a polar molecule — the force that dissolves salt in water.

Why size and polarizability matter

LDFs are not fixed — they grow with the number of electrons and the size of the electron cloud. A big, diffuse cloud is easily distorted, or polarizable, so it forms larger temporary dipoles. That is why dispersion forces climb steadily down a group and with molar mass: I₂ is a solid while F₂ is a gas, even though both are nonpolar and share only LDFs.

IMF strength ranking (per interaction)

ion–dipole > hydrogen bonding > dipole–dipole > London dispersion

A caution: in large molecules, many LDFs can add up to outweigh a single dipole–dipole attraction — compare total forces, not just the type.

Worked example

Rank F₂, Cl₂, and Br₂ by boiling point, highest first, and justify it.

1.All three are nonpolar diatomic halogens, so their only IMF is London dispersion.
2.LDF strength grows with the number of electrons and cloud size (polarizability): Br₂ has the most electrons, F₂ the fewest.
3.Stronger LDFs mean more energy is needed to separate the molecules, so boiling point rises with size.
4.Order: Br₂ > Cl₂ > F₂ (indeed Br₂ is a liquid, Cl₂ and F₂ are gases at room temperature).

Answer: Br₂ > Cl₂ > F₂

Watch out

Hydrogen bonding requires H bonded directly to N, O, or F — not just any hydrogen. CH₄ has hydrogens but no hydrogen bonding, because C is not electronegative enough. HCl is polar but only dipole–dipole; H is on Cl, not N/O/F.

Checkpoint

Which of these substances can form hydrogen bonds between its own molecules?

Checkpoint

CH₄ and Kr have nearly identical molar masses (16 vs 84 aside, both small nonpolar species). Between two nonpolar substances of different size, which has the stronger London dispersion forces?

On the exam

On free-response, always name the IMF and the reason. "Water has hydrogen bonding, which is stronger than the dipole–dipole forces in H₂S, so water has the higher boiling point" earns the point; just saying "water boils higher" does not.

Answer the 2 checkpoints as you read.