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1. (viii) Crystalline solids are generally bounded by flat surfaces.
(ix) Crystalline solids are practically incompressible.
(x) Crystalline solids are considered as true solids.
(xi) Crystalline solids give a clean surface after cleaving them with a knife rather than an
irregular breakage.
Point to Remember
Crystalline solids are those substances whose constituent particles possess a regular
orderly arrangement.
Classification of Crystalline Solids: Various categories of the crystalline solids are
classified depending upon the type of constituent particles and the nature of
intermolecular forces between them.
Broadly, the crystalline solids are divided into four categories.
(i) lonic solid
(ii) Molecular solid
(iii) Covalent or Network solid
(iv) Metallic solid
Ionic solids are solids consisting of oppositely charged ions. They are composed of
positively charged cations and negatively charged anions. These ions are held together by
electrostatic forces. In the case of sodium chloride (NaCl) ionic solid is composed of
simple ions whereas in the case of ammonium nitrate (NH4NO3) ionic solid is composed
of polyatomic ions with NH4
+
and NO3
−
ions. Some characteristics of ionic solids are
 they possess high melting point and boiling point due to strong electrostatic force
of attraction.
 they are usually soluble in a polar solvent but not in a non-polar solvent.
 they are basically insulators of electricity as the ions are not free to move. But
they become good conductors in an aqueous solution as the ions become free.
 they are hard and brittle because their stability depends upon the preservation of
their geometric pattern.
 Molecular solids are solids consisting of molecules. They are further divided into
three types depending upon the nature of the molecules.
(i) Non-polar molecular solids contain non-polar molecules like H2, Cl2, etc. or
noble gas molecules. Weak Van der Waal’s force or dispersion force is the
attractive force between them and due to this; these molecules have low melting
and boiling points. These molecules are soft in nature and are insulators of
electricity.

2. (ii) Polar molecular solids contain polar molecules like HCl, SO2 etc. Strong
dipole-dipole force of attraction between these molecules is responsible for these
solids to be soft, non-conductors of electricity. These solids exist in gaseous or
liquid form at room temperature and pressure with melting and boiling points
higher than nonpolar molecules.