Intermolecular forces, their importance, nature of bonding and their properties

 Intermolecular forces

The forces of attraction between molecules are called intermolecular forces. OR

The forces of attraction between positive end of one molecule and negative end of other molecule are called intermolecular forces. These forces are weaker than ionic or covalent bond. Here, we will discuss two types of intermolecular forces.

1. Dipole-Dipole forces

2. Hydrogen bonding 

Dipole-Dipole forces

When two identical atoms share an electron pair, both atoms exert same force on the shared pair of electrons. Such covalent bond is called non-polar covalent bond. For example, bond in H-H, O=O, N2, etc. 

On the other hand, when two different atoms share an electron pair, both atoms exert different forces of attraction  on the shared pair of electrons. The more electronegative atom pulls the shared pair of electrons more towards itself with greater force than other. So electrons are partially shifted towards more electronegative atom. Thus more electronegative atom becomes partially negatively charged and other atoms becomes partially positively charged. Thus bond between different atoms becomes polar bond. Thus forces of attraction between molecules containing polar bonds are called Dipole-Dipole forces.  For example, H-Cl  H-CN, etc.

Hydrogen bonding 

The force of attraction between highly electron deficient hydrogen atom and lone pair of electrons on a highly electronegative atom such as N, O or F is called hydrogen bond. This phenomenon is called hydrogen bonding. 

Importance of intermolecular forces 

These intermolecular forces are very important in determining properties of water, Biological molecules such as protein,  DNA etc, synthetic material such as glue, paints, resins, etc.

The adhesive action of paints and dyes is developed due to hydrogen bonding.  

Synthetic resins (مصنوعی رال) bind two surfaces together by hydrogen bonding or Dipole-Dipole interactions. 

Nature of Bonding and Properties 

Compounds that consist of ions joined together by electrostatic force of attraction are called ionic compounds. At room temperature,  most of ionic compounds are crystalline solids. 

Structure of NaCl

NaCl is a Colourless cubic crystal.

Each Na+ is surrounded by six Cl- ions and each chloride is surrounded by six Na+ ions. 

There are large attractive forces in crystals of NaCl.

It's melting point is very high. 

Structure of CsCl

CsCl is a Colourless cubic crystal.

Each Cs+ is surrounded by eight Cl- ions and each chloride is surrounded by eight  Cs+ ions. 

here are large attractive forces in crystals of CsCl.

It's melting point is very high. 

Question: How do  ionic compounds like NaCl conduct electricity?

Answer. 

Ionic compounds conduct  electricity only in molten form or in aqueous solution form.

For example, melted NaCl conduct electricity because in molten form Na+ and Cl- are free to move. When a voltage is applied, Na+ ions move towards negative electrode and at the same time Cl- ions move towards positive electrode. This movement of ions is responsible for flow of electricity between the electrodes in external wire.

Ionic compounds also conduct electricity in solution form. This is because  when ionic compound is dissolved in water, the ions become free to move in aqueous medium.

Question: How do ionic compounds not conduct electricity in solid form?

Ionic compounds don't conduct electricity in solid form. This is because of fact that in solid form ions are tightly packed together through strong ionic bonds and aren't free to move .