Induced-dipole attraction

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Induced-dipole attraction (also Wan der Waals forces) involve a polarized molecule, inducing a dipole in another unpolarized molecule, creating an intermolecular attraction. The dipole on the first molecule can originate through an instantaneous dipole process.

This occurs when a molecule with a dipole approaches a molecule without one:

 (+ve) H-Cl (-ve)   Cl-Cl

The polarized molecule repels electrons in the electron cloud of the unpolarized. The negative charge therefore moves towards the other side of the unpolarized molecule. This leaves one side slightly negative, another side slightly positive - it has been polarized:

 (+ve) H-Cl (-ve)   (+ve) Cl-Cl (-ve)

Now the positive side of the Cl-Cl molecule is attracted to the negative Cl of the H-Cl molecule.