Ionic solids are those crystalline solids in which the particles forming the crystal are positively and negatively charged ions, held together by strong electrostatic forces of attraction (ionic bonds). Examples:
The crystals of NaCl, KBr etc. are ionic solids.
Properties of ionic Solids
(1) Physical State:
A well-defined geometrical pattern, describe the arrangements of the cations and anions. So they are crystalline solids at room temperature. Under ordinary conditions of temperature and pressure they never exist in the form of liquids or gases.
(2) High Melting and Boiling Points:
Ionic solids are very stable compounds. They gain the maximum stability. Very high energy is required to separate the cations and anions from each other against the forces of attractions. That is why ionic solids are very hard, have low volatility and high melting and boiling points.
(3) Non-Directional Close Packing:
In Ionic solids their cations and anions attract each other and these forces are non-directional. The packing of the ions arrange themselves systematically in alternate manner.
The structure of the ionic crystals depends upon the radius ratio of cations and anions. For example NaCl and CsF have the same geometry because the radius ratio in both the cases is the same.
(5) Formula Mass:
In the case of ionic crystals we always talk about the formula mass of these substances and not the molecular weight, because they do not exist in the form of molecules.
Ionic crystals do not conduct electricity in the solid state because on account of electrostatic force existing between them the cations and anions remain tightly held together with each other and hence occupy fixed positions. Ionic crystals conduct electricity when they are in solution or in the molten states. In both cases ions become free.
Ionic crystals are highly brittle because ionic solids contain parallel layers having cautions and anions in alternate positions. They have high density due to close packing of ions. They give fast ionic reactions.