12th chemistry solid state - Amorphous and Crystalline Solids & Classification of Crystalline Solids

Amorphous and Crystalline Solids & Classification of Crystalline Solids

Amorphous and Crystalline Solids

• Based on the nature of the order of arrangement of the constituent particles, solids are classified as amorphous and crystalline.
• Differences between amorphous and crystalline solids are listed in the given table.

Amorphous solids		Crystalline solids
1	Have irregular shape	1	Have definite characteristic geometrical shape
2	Have only short-range order in the arrangement of constituent particles	2	Have long-range order in the arrangement of constituent particles
3	Gradually soften over a range of temperature	3	Have sharp and characteristic melting point
4	When cut with a sharp-edged tool, they cut into two pieces with irregular shapes	4	When cut with a sharp-edged tool, they split into two pieces with plain and smooth newly generated surfaces.
5	Do not have definite heat of fusion	5	Have definite and characteristic heat of fusion
6	Isotropic in nature	6	Anisotropic in nature
7	Pseudo solids or super-cooled liquids	7	True solids

Classification of Crystalline Solids

• Based on the nature of intermolecular forces, crystalline solids are classified into four categories −

• Molecular solids
• Ionic solids
• Metallic solids
• Covalent solids

• Molecular solids

• Constituent particles are molecules

• Ionic solids
  • Constituent particles are ions
  • Hard but brittle
  • Insulators of electricity in solid state, but conductors in molten state and in aqueous solution
  • High melting point
  • Attractive forces are Coulombic or electrostatic
  • Example − NaCl, MgO, ZnS
• Metallic solids
  • In metallic solids, positive ions are surrounded and are held together in a sea of delocalised electrons.
  • Hard but malleable and ductile
  • Conductors of electricity in solid state as well as molten state
  • Fairly high melting point
  • Particles are held by metallic bonding
  • Example − Fe, Cu, Mg
• Covalent or network solids
  • Constituent particles are atoms
  • Hard (except graphite, which is soft)
  • Insulators of electricity (except graphite, which is a conductor of electricity)
  • Very high melting point and can decompose before melting
  • Particles are held by covalent bonding
  • Example − SiO₂ (quartz), SiC, diamond, graphite

Add to your knowledge

The property by virtue of which two or more crystalline solids having similar chemical composition exist in the same crystalline form is called isomorphism. For example: Na3PO4. The property by virtue of which a particular substance exists in more than one crystalline form is called polymorphism. For example: existence of calcium carbonate in two crystalline forms called calcite and aragonite.