Friday, July 28, 2017


There are certain forces that condense matter.These forces can be both bonding forces and non bonding forces which are called intramolecular forces and Intermolecular forces respectively.
(I) Intramolecular Forces(bonds that exists within the molecules) : Intramolecular forces are the chemical forces that operate by the formation of bonds between the atoms to form molecules. Example Ionic bond, Covalent bond which we have studied in the previous chapter.
(II) Intermolecular forces(forces that exists between molecules) : Intermolecular forces are the physical forces or just the interactions which act between the neighbouring bonded molecules. The intermolecular forces are weaker than the intramolecular forces.
The forces of attraction and repulsion between the intracting particles are called intermolecular forces. These forces hold the molecules together which are covalently bonded. Intermolecular forces are not encountered in systems that employ ionic bonding.
Van der waals, a Dutch scientist explained that attractive forces present between molecules lead to deviation of real gases from the ideal gas behaviour .So, to honour the scientist, intermolecular forces are also known as van der waal forces. These forces of attraction exists between polar as well as non-polar molecules. These are the electrostatic forces of attraction that exist between an area of negetive charge on one molecule and an area of positive charge on a second molecule. There are three types of vanderwaal forces or interactions.
(i) Dispersion forces or London forces.
(ii) Dipole-dipole forces.
(iii) Dipole- induced dipole forces
Fourth type is particularly strong type of dipole -dipole interactions called hydrogen bonding.
(i) Dispersion forces or London forces:(associated with non-polar structures) These forces are present in non-polar molecules like H2 ,O2 and N2 are also in nonpolar monoatomic molecules such as noble gases like He, Ne, Ar etc., which exist with intermolecular forces and no bonding at all.
These non -polar molecules are electrically symmetrical, so there is no dipole moment. But sometimes the temporary dipoles can be formed. The electrons of a neutral molecule keep on oscillating with respect to the nuclei of the atoms. As a result of this, at a given instant, positive charge may be concentrated in one region of the molecule . Thus the non-polar milecule develops momentarily dipole due to unsymmetrical electronic charge distribution. Now, this polarised molecule distorts the electron density of tge neighbouring molecule. These interactions are therefore also known as induced dipole induced dipole interactions .Now the attraction between the two oppositely charged ends of two neighbouring molecules attract each other and this type of force of attraction is called London force after the name of German Physicist Fritz London who proposed this type of force of interaction .This force is also known as dispersion force.
London forces are attractive in nature and the interaction energy is inversely proportional to the sixth power of distance between two interacting particles.
These forces work only at short distances of approx.  500 pm.
The magnitude of the forces depends upon the polarisability of the atom or molecule.
Polarisability is the ease with which the electrons and nuclei can be displaced from their average positions. Larger the polarisability, stronger are the London forces.
London forces directly proportional to polarisability.