Polarization – The asymmetrical distribution of electron clouds between two atoms involved in a chemical bond due to the difference in electronegativities of the two atoms concerned or due to another external influence is known as polarization.
Dipole moment
The product of the charge ( δ ) present on individual atoms in such a molecule and the length of the bond between the atoms is called the dipole moment ( µ = δ × r ) of the bond. In polyatomic molecules, the dipole moment is considered for each bond. Their resultant is taken as the dipole moment of the molecule.
The resultant of the dipole moments of all the N- H bonds gives the dipole moment of the NH3 molecule and the resultant of the dipole moments of all the N- F bonds gives the dipole moment of the NF3 molecule.
• For some symmetric molecules the dipole moment is zero
All types of intermolecular interactions exist among molecules are commonly referred as van der Waals interactions. Those interactions can be categorized into five types. Ion – dipole interactions Dipole – dipole interactions and hydrogen bonds Ion – induced dipole interactions Dipole – induced dipole interactions Dispersion (London) forces
Ion – dipole interactions
These interactions are said to take place when polar molecules are attracted to a cation or an anion. The strength of these interactions depend on the charge and size of the ions and on the magnitude of the dipole.
Dipole – dipole interactions
These forces are found in polar molecules which have permanent dipole
Hydrogen bonds
Hydrogen bond is a special type of electrostatic interaction between a hydrogen atom covalently bonded to an electronegative element (X) and another electronegative element (Y) such as fluorine or oxygen or nitrogen which has one or more lone pairs. This interaction can be represented by Xδ-—Hδ+ – – – – Yδ-
Existence of hydrogen bonding can be proved by referring to the variation of boiling temperatures of group 15, 16 and 17 hydrides.
Ion – induced dipole interactions
Dipole – induced dipole interactions
This type of attraction is found between an uncharged non-polar and a polar species.
eg. Dissolving non-polar species such as O2 , I2 , Xe, etc. in water.
Dispersion forces (London forces or London dispersion forces)
Interactions between non-polar molecules or atoms are referred as dispersion forces. Any non-polar molecule can be temporary polarized due to an instantaneous deformation of its electron cloud. Due to the polarity of a such molecule another non polar molecule can also be temporary polarized. Interactions between such molecules are referred as dispersion forces. Such forces exist between any molecules.
Generally dispersion forces are the weakest among all types of van der Waals forces. However, there are instances where strength of dispersion forces exceeds dipole – dipole interactions. eg – Compound Melting point Nature of secondary interaction CH3 F -142 °C Dipole-dipole interactions and dispersion forces CCl4 -23 °C Dispersion forces
