Intermolecular force

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Intermolecular and interatomic forces are electromagnetic forces which act between molecules. There are a number of types including:

  • dipole-dipole attraction
  • instantaneous dipole attraction
  • induced-dipole attraction
  • instantaneous-dipole induced-dipole attraction
  • hydrogen bond attractions

Dipole-dipole attraction

A dipole-dipole attraction involves two molecules with dipoles. An area of the molecule with a charge is attracted to another molecule with the oppposite charge:

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

The larger the difference between the two charges then the stronger the bond.

Instantaneous dipole attraction

Instantaneous dipole attraction involves molecules without a permanent dipole. Molecules can be thought to be surrounded by a negatively charged cloud of constantly moving electrons. If at any particular instant more of the electrons happen to be more at one side of the molecule than another, this side becomes more negative, whereas the other side becomes more positive - a dipole is developed.

Although this dipole is only evident for a very short time, it can cause intermolecular attractions to occur through induced-dipole attraction.

Induced dipole attraction

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.

Instantaneous-dipole induced-dipole attraction

An Instantaneous-dipole-induced-dipole bond (or Wan der Waal force) is an attraction between two unpolarized molecules. Their are three stages involved in this kind of attraction. First a dipole instaneously happens (1), then this dipole is induced to the second molecule (2). Once both molecules have dipoles, they are attracted to each other (3).

This type of attraction is involved in polymer chains and determines how dense and strong they are.