States of Matter
The forces of attraction and repulsion between particles (atoms and molecules) are called intermolecular forces. This term does not include electrostatic forces (existing between two oppositely charged ions) and the forces in covalent bonds.
Van der Waals Forces: Attractive intermolecular forces are known as van der Waals forces. These forces include dispersion forces (or London forces), dipole-dipole forces, and dipole-induced dipole forces.
Dispersion Forces or London Forces: The force of attraction between two temporary dipoles is called London force. It is also called dispersion force. These forces are always attractive. The interaction energy of this force is inversely proportional to the sixth power of the distance between two interacting particles. (`1/r^6`). London forces are important only at short distances (about 500 pm) and their magnitude depends on the polarisability of the particle.
Atoms and non-polar molecules are electrically asymmetrical. As their electronic charge cloud is symmetrically distributed, they have no dipole moment. But a dipole may develop momentarily even in such atoms and molecules.
Let us assume there are two atoms A and B in close vicinity to each other. The electronic charge distribution in one of the atoms (A) becomes asymmetrical for a moment. It means the charge cloud is more on one side than on the other side. This results in development of instantaneous dipole of atom A for very short time. This transient dipole distorts the electron density of atom B, and as a consequence a dipole is induced in atom B. The temporary dipoles of atoms A and B attract each other.
These forces act between the molecules which possess permanent dipole. Ends of the dipole possess ‘partial charges’ and these charges are shown by Greek letter delta (δ). Partial charges are always less than unit electronic charge (1.6 × 10-19).
Dipole-dipole interaction energy between stationary polar molecules is proportional to `1/r^3` and the energy between rotating polar molecules is proportional to`1/r^6`, where r is the distance between polar molecules. Dipole-dipole interaction is stronger than London forces but is weaker than ion-ion interaction because only partial charges are involved in dipole-dipole interaction.
Dipole-Induced Dipole Forces
This is an attractive force and is present between the polar molecule having permanent dipole and the molecule lacking permanent dipole. Permanent dipole of the polar molecule induces dipole on electrically neutral molecule by deforming its electronic cloud.
The interaction energy is proportional to `1/r^6`. Induced dipole moment depends on the dipole moment in permanent dipole and polarisability of electrically neutral molecule.
This is a special case of dipole-dipole interaction. Energy of hydrogen bond varies between 10 to 100 kJ mol-1. This is a significant amount of energy, hence hydrogen bonds are powerful force in determining the structure and properties of many compounds.
Note: Apart from the attractive forces discussed above, repulsive forces are also present between molecules. When two molecules are brought closer to each other, repulsion between electron clouds and that between the nuclei of two molecules come into play. Magnitude of repulsion rises rapidly with decrease in distance between molecules. Due to this, it is difficult to compress liquids and solids.
The energy of a body arising from motion of its atoms or molecules is called thermal energy. Thermal energy is directly proportional to the temperature of the substance. Thermal energy gives the measure of average kinetic energy of particles and is thus responsible for movement of particles. This movement of particles is called thermal motion.
Intermolecular Forces Vs Thermal Interactions
Three states of matter are the result of balance between intermolecular forces and thermal energy of molecules. Intermolecular interaction is the least in gases and highest in solids. Thermal energy is the least in solids and highest in gases. When molecular interactions are very weak, molecules do not cling together to make liquid or solid unless temperature is lowered to reduce thermal energy.