Classification of Crystalline Solids

A crystal or crystalline solids is a solid material which consists of a large number of small Crystal. Each small crystal has a definite characteristic geometrical shape. Arrangement of constituent particles is ordered and repetitive in three dimensions. It has a sharp melting point. Crystalline solids are anisotropic in nature. Sodium chloride and quartz are examples of crystalline solids.

Classification of Crystalline Solids

On the basis of nature of intermolecular forces or bonds that hold the constituent particles together, it is classified into four categories.

1.Molecular solids 

Molecules are the constituent particles of molecular solids, so these are subdivided into three categories.

• Non-polar molecular solids: They comprise atoms. For example, argon and helium or the molecules formed by non-polar covalent atoms. The solids are soft and non-conductors of electricity. They have low melting points and are usually in liquid or gaseous state at room temperature.

• Polar molecular solids: The molecules are held by strong dipole-dipole interactions. These solids are soft and non-conductors of electricity. They have high melting points than non-polar molecular solids. Most of these are gases or liquids at room temperature and pressure.

• Hydrogen bonded molecular solids: These solids contain polar covalent bonds between H and F, O or N atoms. They have a strong hydrogen bond. They are non-conductors of electricity. They are generally volatile liquids or soft solids under room temperature and pressure.

2.Ionic solids 

Ions are the constituent particles of ionic solids. They are formed by the three dimensional of cations and anions bound by strong coulombic forces. These solids are hard and brittle in nature. They have high melting and boiling points. They are insulators in the solid-state but conduct electricity when dissolved in water.

3.Metallic solids

Metals are the collection of positive ions surrounded by and together by a sea of free electrons. These electrons are mobile. These free electrons are responsible for the high electrical and thermal conductivity of metals. They are lusture, highly malleable and ductile.

4.Covalent or network solids

Most of the crystalline solids of non-metallic result from the formation of covalent bonds between adjacent atoms throughout the crystal. They are also called giant molecules. Covalent bonds are strong and directional in nature. They are very hard and brittle. They have extremely high melting points and may even decompose before melting. They are insulators. Diamond and silicon carbide are examples. Although Graphite also belongs to this crystal, it is soft and is a conductor of electricity. This is due to its typical structure.