Permanent dipole interactions occur between permanent dipoles with regions of both positive and negative partial charges. They arise from electronegative atoms like oxygen, nitrogen and the halogens.
London dispersion forces, also known as van der Waals dispersion forces, are intermolecular interactions which arise due to temporary fluctuating dipoles. When molecules get close to one another, these temporary dipoles become induced dipoles, with δ⁺ attracting δ⁻, resulting in an intermolecular interaction. Individually they're weak, but their strength increases with number.
Hydrogen bonding is the strongest intermolecular interaction. It occurs between polarised hydrogen atoms and electronegative atoms.
Intermolecular interactions are weak in comparison to the strength of interatomic bonds, but collectively they contribute greatly to the physical properties of a molecule (ie. boiling point).
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