Within or between molecules?
Intramolecular forces act within a molecule. In the substances considered here, these are the covalent bonds joining atoms together.
Intermolecular forces are attractions acting between separate molecules. They are weaker than the covalent bonds within the molecules.
| Type | Where it acts | Examples |
|---|---|---|
| Intramolecular | Between atoms within one molecule | Single, double, triple and coordinate covalent bonds |
| Intermolecular | Between neighbouring molecules | Induced dipole–dipole, permanent dipole–dipole and hydrogen bonding |
The three intermolecular forces
Induced dipoles
Temporary changes in electron distribution create dipoles that induce dipoles in neighbouring particles.
Permanent dipoles
Polar molecules attract one another through their permanent and regions.
Hydrogen bonding
A particularly strong intermolecular attraction occurs when suitable N–H, O–H or F–H bonds and lone pairs are present.
Induced dipole attractions are present in molecular substances even when other types of intermolecular force are also present. Permanent dipole attractions and hydrogen bonding therefore act in addition to induced dipole forces where the necessary molecular features occur.
Exam Tip: When explaining the boiling of a molecular substance, refer to overcoming intermolecular forces between molecules. Do not say that the covalent bonds within the molecules are broken.