Intermolecular Forces

Intermolecular forces are weak forces between molecules. The strength of these forces is significantly less than the strength of typical chemical (molecular) bonds or electrostatic (ionic) interactions. Their importance lies in the fact that intermolecular forces are an additional source of stabilization.

Intermolecular forces cause molecules to become liquids and solids. Stronger intermolecular forces result in:

  1. Higher melting points and higher boiling points.
  2. Lower vapor pressure of a liquid.
  3. Increased surface tension

Types of Intermolecular Forces

From strongest to weakest, the intermolecular forces that we will consider are:

  1. Hydrogen Bonding is the strongest type of intermolecular force we will consider. Hydrogen bonding requires a Donor (either H--F, H--O, or H--N) and an Acceptor (either :F, :O, or :N).
  2. Dipole-Dipole Interactions are the result of attraction between polar molecules. In polar molecules, one end of the molecule has a partial positive charge (δ+), while another portion of the molecule has a partial negative charge (δ-). These opposite partial charges in different molecules attract.
  3. London Forces (also called van def Waal's forces) are due to the instantaneous position of electrons within a molecule. As the number of electrons increases, the strength of these London forces also increases.