Question

Based on the intermolecular forces of attraction,how can we classify solids as Ionic,Covalent,Molecular or Metallic and in Molecular as polar,non polar and hydrogen bonded?

Answer 1

On the basis of intermolecular forces solids can be classified as follows:

Molecular Solids

Solid composed of molecules as constituent particles. These solids can further be categorized into following types:

• Non polar Molecular Solids:
• They are composed of either atom.
• H2, Cl2 and I2.are some of the typical example.
• In non-polar Molecular solids the comprised atoms or molecules are held together by weak dispersion forces or London forces.
• They are soft and non-conductors of electricity.
• They have low melting points and usually exist in liquid or gaseous state at room temperature and pressure.

(ii) Polar Molecular Solids:

• The molecules like HCl, SO2, are formed by polar covalent bonds.
• In polar Molecular solids the comprised atoms or molecules are held together by stronger dipole-dipole interactions.
• They are soft and non-conductors of electricity.
• The melting points of these solids are higher than those of non-polar molecular solids and usually exist in liquid or gaseous state at room temperature and pressure.
• Molecules like SO2 and solid NH3 are some examples of such solids.

(iii) Hydrogen Bonded Molecular Solids:

• Hydrogen Bonded Molecular Solids contain polar covalent bonds between H and F, O or N atoms.
• Strong hydrogen bonding binds molecules of such solids like H2O (ice).
• They are non-conductors of electricity and are volatile liquids or soft solids under room temperature and pressure.

Ionic Solids

• The constituent particles of these solids are ions.
• The solid is composed of three dimensional arrangements of cations and anions that are bonded by strong coulombic (electrostatic) forces.
• These solids are hard and brittle in nature and have high melting and boiling points.
• They are electrical insulators in solid state due to the absence of movement of free electrons but are good conductors in molten state due to the movement of ions.

Metallic Solids

• In these solids positive ions is surrounded by mobile free electrons and are evenly spread all over the crystal.

• Each metal atom donates one or more electrons to the group of mobile electrons which increases the electrical and thermal conductivity of the metallic elements.
• Application of electric field makes these electrons flow through the linkage of positive ions.
• Whereas application of heat to one portion of a metal makes the thermal energy spread uniformly throughout by free electrons.
• Presence of free electrons in metals makes them lustrous, malleable and ductile. For example, Cr, Fe etc.

Covalent solids

• Crystalline solids of non-metals comprises of covalent bonds between adjacent atoms that are strong and directional in nature due to which atoms are held very strongly at their positions all over the crystal.
• They are also called giant molecules.
• These solids are very hard and brittle.
• They have extremely high melting points and may decompose before melting.
• They are insulators and do not conduct electricity.
• Diamond and silicon carbide are typical examples of such solids but Graphite is an exception as it is soft and a good conductor of electricity.