secondary interactions existing in various systems

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polarization, dipole moment and polarizability  

• The asymmetrical distribution of electron clouds between two atoms involved in 

a chemical bond due to the difference in electro negativities of the two atoms concerned 

or due to another external influence is known as polarization. 

Example : 

 

  

• 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. 

• For some symmetric molecules the dipole moment is zero. 

Example: 

  

 

Secondary interactions 

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 

Ion - induced dipole interactions 

When a charged particle such as an ion is introduced into the neighborhood of an uncharged non-polar molecule ( eg. an atom of a noble gas) it will distort the electron cloud of the atom or molecule. 

eg.I2 dissolved in aqueous KI to form 

I−3  ion 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. 

Boiling point / 0C 

Hydrogen bonds 

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.  

Compound	Melting point	Nature of secondary interaction
CH3F	-142 0C	Dipole-dipole interactions  and dispersion forces
CCl4	-23 0C	Dispersion forces

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