245. How the crystalline solids are classified on the basis of the nature of bonding? Give suitable examples and nature of the forces present in different types of solids.

i) A crystalline solid usually consists of a large number of small crystals, each of them having a definite Characteristic geometrical shape. In a crystal, the arrangement of constituent particles (atoms, molecules or ions) is ordered. It has long range order which means that there is a regular pattern of arrangement of particles which repeats itself periodically over the entire crystal. Sodium chloride and quartz are typical examples of crystalline solids.

ii) Crystalline solid have a sharp melting point.

Crystalline solids can be classified on the basis of nature of intermolecular forces operating in them into four categories.
i) molecular
ii)ionic
iii)metallic
iv)covalent solids.

i) molecular: Molecules are the constituent particles of molecular solids. These are further sub divided into the following categories:

a) Non polar Molecular Solids: They comprise of either atoms, for example, argon and helium or the molecules formed by non polar covalent bonds for example H₂, Cl_2.

b) Polar Molecular Solids: The molecules in which solids are held together by relatively stronger dipole-dipole interactions.for example HCl, SO₂, etc.

c) Hydrogen Bonded Molecular Solids: The molecules of such solids contain polar covalent bonds between H and F, O or N atoms. Strong hydrogen bonding binds molecules of such solids like H₂O (ice).

ii) Ionic solid : Ions are the constituent particles of ionic solids. In ionic solid cations and anions bound by strong coulombic (electrostatic) forces. for example NaCl, KCl etc.

iii) metallic solid: These solids contain metal atoms as constituent particles. As metals have a good tendency to lose their valence electron and change in to positively charged metal ions (kernel). These electrons can easily move throughout the whole crystal and form the sea of free electrons. for example iron, calcium etc.

iv) covalent solids: A wide variety of crystalline solids of non-metals result from the formation of covalent bonds between adjacent atoms throughout the crystal. They are also called giant molecules. for example diamond, silicon carbide, etc.