We have studied that in the covalent molecules, the constituent atoms are held by the covalent bonds. These molecules may be polar or non-polar. In a covalent substance, the covalent molecules are further held by some attractive forces. For example, H20 molecule is a discrete unit which contains two 0-H covalent bonds. But a sample of water contains large number of H20 units held by some attractive forces. The attractive forces operating between the molecules of a given substances (solid, liquid or gas) are called intermolecular forces.
TYPES OF INTERMOLECULAR FORCES
The different types of intermolecular forces among the covalent molecules are:
1. Hydrogen bonds
These attractive forces operate among the molecules containing 0-H or N-H or F-H
2. Van der Waals’ forces
These attractive forces operate among the polar as well as non polar molecules. They got their name from Dutch scientist Johannes van der Waal (1837 -1923) who first suggested their nature.
van der Waals’ forces are further classified as:
(i) Dispersion forces or London forces
(ii) Dipole-Dipole forces and
(iii) Dipole-induced dipole forces.
It is worth noting that the electrostatic forces between the ions of opposite charges (interionic forces) and those between ions and polar molecules (ion-dipole forces) do not fall in the category of van der Waal forces.