Polar molecules have partial negative and partial positive charge or a dipole, Dipole-dipole forces act between molecules which are neutral but possess permanent dipole. The separation of charges depends on the electronegativity of the bonded atom. For example, Dipole-Dipole forces are present between two HCl molecules. Chlorine is more electronegative so pull half the pair of electron towards itself so chlorine has partial negative charge and hydrogen has partial positive charge.
Dipole-dipole forces are stronger than London forces because permanent dipoles are involved but weaker than ion-ion interaction because partial charges present in polar molecules are always less than unit electric charge (1.6Х10-19) present on ions.
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