Intermolecular Forces

What are intermolecular forces?

Intermolecular forces are the forces of attraction and repulsion that act between interacting particles — atoms or molecules. This is a narrower idea than it sounds: it deliberately excludes two things you already know about —

• the strong electrostatic force that holds two oppositely-charged ions together, and
• the covalent bond that holds the atoms of a single molecule together.

Both of those are much stronger, intramolecular interactions. Intermolecular forces are the weaker forces that act between separate molecules or atoms, and they are what decide whether a substance is a gas, a liquid, or a solid at a given temperature.

van der Waals forces

The attractive intermolecular forces collectively carry the name van der Waals forces, honouring the Dutch physicist Johannes van der Waals (1837–1923), who used them to explain why real gases deviate from ideal behaviour (a topic we return to later in this unit). van der Waals forces are not one single force — they cover a family of interactions that differ a lot in strength:

• Dispersion (London) forces
• Dipole–dipole forces
• Dipole–induced dipole forces

A particularly strong member of the dipole-dipole family is hydrogen bonding. Because only a handful of elements can take part in it, hydrogen bonding is usually treated as its own category rather than folded into "ordinary" dipole-dipole attraction — you met this interaction already in Unit 4.

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Careful distinction: the attraction between an ion and a nearby dipole (an ion-dipole force) is not classified as a van der Waals force, even though it is also an intermolecular attraction. Keep this apart from the four van der Waals-type forces above, which we now examine one by one.