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Crystalline Solids: Molecular and Ionic Solids

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Crystalline Solids: Molecular and Ionic Solids - Lesson Summary

On the basis of the type of forces holding the constituent particles together, crystalline solids are broadly classified into molecular solids, ionic solids, metallic solids, and covalent or network solids.
 
Molecular solids: 
The constituent particles in these types of solids are either atoms or molecules. The atoms or molecules are held together by weak forces of attraction. Molecular solids are divided further into non-polar molecular solids, polar molecular solids, and hydrogen-bonded molecular solids.
 
  Molecular Solids
→ Non Polar molecular Solids
→ Polar Molecular Solids
→ Hydrogen Bonded Molecular Solids
 
Non-polar molecular solids:  
The constituent particles are either atoms or molecules. The atoms or molecules in these solids are held together by weak dispersion or London forces. These solids are non- conductors of electricity.
 
Polar molecular solids: 
The constituent particles – molecules-are formed by polar covalent bonds.
EX: Solid sulphur dioxide and solid hydrochloric acid.

The molecules in polar molecular solids are held together by relatively stronger dipole-dipole interactions. These solids have relatively high melting and boiling points. These solids are soft and non-conductors of electricity.
 
Hydrogen-bonded molecular solids: 
The constituent particles – molecules - are formed by polar covalent bonds. the bonded atoms in these molecules are essentially hydrogen and a highly electro-negative atom such as fluorine, oxygen or nitrogen.
EX: Ice and solid hydrofluoric acid

The molecules in these solids are held together by strong forces of attraction called hydrogen bonds. These solids have high melting points. These solids are also non-conductors of electricity.
 
Ionic solids
These solids have the ions as their constituent particles. The columbic or electrostatic forces of attraction hold the ions - cations and anions - together in the crystal.
EX: sodium chloride, caesium chloride and zinc sulphide.

These solids have high melting and boiling points. These solids are non-conductors of electricity in solid state. These solids are good conductors of electricity in fused or in their aqueous solutions.

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