Unit – II A

Classification of Crystalline Solids

Broadly crystalline solids are classified into 4 types.

A) Molecular Solids:

  • Molecules of the same compound are the constituent particles of molecular solids. Depending upon type of molecules involved in crystallisation and the nature of intermolecular forces of attraction between the neighbouring molecules, the molecular solids are further sub divided into the following categories:

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, Cl2 and I2.
  • In these solids, the atoms or molecules are held by weak dispersion forces or London forces.
  • These solids are soft and non-conductors of electricity.
  • They have low melting points and are usually in liquid or gaseous state at room temperature and pressure.

Polar Molecular Solids:

  • The molecules of substances like HCl, SO2, etc. are formed by polar covalent bonds.
  • The molecules in such solids are held together by relatively stronger dipole-dipole interactions.
  • These solids are soft and non-conductors of electricity.
  • Their melting points are higher than those of non polar molecular solids yet most of these are gases or liquids under room temperature and pressure.
  • They possess permanent dipole moment. The molecules in these solids are bonded together by stronger dipole dipole interaction.
  • Solid SOand solid NH3 are some examples of such solids.

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), ammonia.
  • They are non-conductors of electricity.
  • Generally they are volatile liquids or soft solids under room temperature and pressure.
  • Liquids solidify only on cooling.

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