In non-ionic compounds such as organic compounds, the structural units re molecules. The force holding these molecules together are generally very weak and are of three types.

i) Dipole-dipole attraction:- in polar molecules, the positive end of one molecule is attracted by the negative end of another molecule. As a result of this dipole-dipole attraction, polar molecules are held to each other.

ii) Hydrogen bonding:- intermolecular hydrogen bonding brings association of the molecules and thus, increases the melting and boiling points of the compound.

iii) Van Der Waals’ forces:- in non-polar compounds, there is a very weak intermolecular attraction due to electrostatic attraction between the nuclei of one molecule and the electron of the other. This attraction is largely compensated by the repulsion between the electrons of neighbouring molecules as well as the repulsion between the nuclei of neighbouring molecules. The van der Waals’ forces are very close to each other. Van der Waals’ forces are stronger in the compounds of high molecular masses.

The relative order of these intermolecular forces is:- Hydrogen bond>dipole-dipole attraction>van der Waals’ attraction.

Intermolecular Forces:- we have learned that the intermolecular forces play very significant role in the properties of substances. These are weak intermolecular forces which cause deviation from ideal behavior in gases. Solids have also been classified according to the type of intermolecular forces in them. In addition to normal covalent bond, ionic bond and metallic bond there are weak attractive intermolecular forces which occur in all kind of molecular solids. These are present even in case of non polar molecules, in the solid and liquid states. These are classified as:-

(i)          Dipole-dipole forces

(ii)         Dipole-induced dipole forces

(iii)        Instantaneous dipole instantaneous induced dipole forces

(iv)       Hydrogen bonding