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What are intermolecular forces?

The term 'intermolecular forces' is used to describe all the forces, both attractive and repulsive, between groups of neutral molecules or atoms. Forces between atoms within a molecule ('intramolecular forces') are usually called chemical bonds, however there can be forces between atoms in a molecule (particularly large molecules) that are not classified as chemical bonds.

Intermolecular forces are caused by the attractions and repulsions between the charged particles that make up the atoms in molecules. As two molecules approach each other, the negatively charged electrons in one molecule are attracted to the positively charged nuclei in the other molecule. But at the same time these electrons are repelled by the electrons in the other molecule. This complex combination of attraction and repulsion can result in the temporary distortion of the electron cloud around one molecule so that a weak overall attraction exists between the two molecules. This is called a dispersion force and it exists between all molecules.

The picture on the right shows the temporary distortion of the electron cloud around an atom as it approaches another atom. The motions of the electrons in one atom influence the electrons in the other atom (the electron motions are said to be 'correlated') and this give rise to the dispersion force.

If a molecule has an uneven distribution of charge, so that one part of the molecule carries a slight negative charge and another part carries a slight positive charge, the molecule is said to have a dipole moment. If two molecules, each with a dipole moment, come close to each other, then if they are oriented so that the oppositely charged portions are aligned there will be an attraction between them. This is called a dipole-dipole attraction and it operates only between molecules with permanent dipole moments.

The picture on the right shows the distorted electron clouds around two molecules, each having a permanent dipole moment. As two molecules approach, that end of one molecule which has a lower electron density is attracted to the end of the other molecule which is more electron-rich.

A special, and strong, version of the dipole-dipole force operates between molecules which contain a hydrogen atom bonded to either fluorine, oxygen or nitrogen. Atoms of these three elements contain pairs of electrons which are not involved in bonding within the molecule, and these atoms also strongly attract the electrons involved in the bond to the hydrogen atom. This leaves the hydrogen with a relatively large positive charge which allows it to be attracted to the electrons on one of these other atoms located in a nearby molecule. This is called 'hydrogen bonding' and is the strongest of the intermolecular forces.

An understanding of intermolecular forces is important because it allows chemists to predict properties such as melting and boiling points and whether one substance will dissolve in another substance.

If you would like to make any comments about this article, please e-mail the author, Dr Ron Haines.