MCAT Content / Liquid Phase Intermolecular Forces / London Dispersion Forces

London Dispersion Forces

Topic: Liquid Phase Intermolecular Forces

London Dispersion Forces are the weakest intermolecular force and exist when a temporary dipole forms in a non-polar molecule, creating an induced dipole in an adjacent molecule.

Non-polar molecules have an even distribution of electrons due to their atoms having very similar or the same electronegativities. However, sometimes this even distribution can be affected by an adjacent molecule which may causes the distribution to become temporarily uneven, this creates an instantaneous dipole, and the molecule now has partial positive and negative charges (see below).

These instantaneous dipoles can cause dipoles to form in adjacent molecules by repelling and attracting them with electrostatic interactions. This is known as an induced dipole. These interactions are known as London dispersion forces, and are considered part of the van der Waals forces or weak intermolecular attractions.

 


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Key Points

• London dispersion forces are the weakest intermolecular forces arising from induced instantaneous dipoles.

• Induced dipoles occur when there is a temporary uneven distribution of electrons in a non-polar molecule causing repulsion or attraction in an adjacent molecule.


Key Terms

Dipole: Occurs within a molecule when two atoms in a covalent bond have different affinities for electrons (electronegativities), so the ‘push and pull’ of their shared electrons results in one atom maintaining most of the electron density and a partial negative charge, leaving the other atom with a partial positive charge.

Instantaneous dipole: A dipole which forms in a non-polar molecule due to the sudden, temporary, uneven distribution of electrons.

Induced dipole: A dipole formed in a non-polar molecule caused by an adjacent molecule which has an instantaneous dipole.

Van der Waals forces: The sum of the attractive or repulsive forces caused by intermolecular forces.



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