Intermolecular forces are the forces of attraction or repulsion which act between neighboring particles (atoms, molecules or ions). These forces are weak compared to the intramolecular forces—the forces which keep a molecule together. For example, the covalent bond present within HCl molecules is much stronger than the forces present between the neighboring molecules, when the molecules are sufficiently close to each other.
There are four types of attractive intermolecular forces:
- Dipole-dipole forces: electrostatic interactions of permanent dipoles in molecules
- Dipole-induced dipole forces or Debye forces: the attractive interaction between a permanent multipole on one molecule with an induced (by the former di/multi-pole) multipole on another
- Instantaneous dipole-induced dipole forces or London dispersion forces: forces caused by correlated movements of the electrons in interacting molecules
- Ion-dipole forces: discussed below
Ion-dipole and ion-induced-dipole forces operate much like dipole-dipole and induced-dipole interactions. However, ion-dipole forces involve ions instead of solely polar and non-polar molecules. Ion-dipole forces are stronger than dipole interactions because the charge of any ion is much greater than the charge of a dipole moment. Ion-dipole bonding is stronger than hydrogen bonding. An ion-dipole force consists of an ion and a polar molecule interacting. They align so that the positive and negative forces are next to one another, allowing for maximum attraction. An ion-induced dipole force consists of an ion and a non-polar molecule interacting. Like a dipole-induced dipole force, the charge of the ion causes a distortion of the electron cloud in the non-polar molecule (Figure 1).