Intermolecular Forces i (van der Waals forces) Weaker than 偶极 2.Dipole-Dipole Attraction: ion-dipole forces Interaction between polar molecules Dipole-dipole forces among polar molecules cause them to have "preferred"orientations in the liquid or solid states.2. Dipole-Dipole Attraction Dipole Attraction: Interaction between polar molecules Intermolecular Forces I (van der Waals forces) Weaker than ion-dipole forces 偶极 Dipole-dipole forces among polar molecules cause them to have “preferred” orientations in the liquid or solid states