Solid State - Chapter Notes - Class XII

TOP CONCEPTS 

Solid: Solid is a state of matter in which the constituting particles are arranged very closely. The constituent particles can be atoms, molecules or ions.

Properties of solids: 

1. They have definite mass, volume and shape. 
2. Intermolecular distances are short and hence the intermolecular forces are strong. 
3. Their constituent particles have fixed positions and can only oscillate about their mean positions. 
4. They are incompressible and rigid.

Classification of on the basis of the arrangement of constituent particles: 

• Crystalline solids: The arrangement of constituent particles is a regular orderly arrangement. Example: iron, copper, diamond, graphite 
• Amorphous solids: The arrangement of constituent particles is an irregular arrangement. Example: Glass, plastics, rubber

Properties of crystalline solids: 

1. They have a definite characteristic geometrical shape 
2. They have a long range order.  
3. They have a sharp melting point.  
4. They are anisotropic in nature i.e. their physical properties show different values when measured along different directions in the same crystal. 
5. When cut with a sharp edged tool, they split into two pieces and the newly generated surfaces are plain and smooth. 
6. They have a definite and characteristic heat of fusion  
7. They are called true solids

Polymorphic forms or polymorphs: The different crystalline forms of a substance are known as polymorphic forms or polymorphs. For example: graphite and diamond.

Characteristics of amorphous solids:

1. They have an irregular shape. They have a short range order.
2. They gradually soften over a range of temperature.
3. They are isotropic in nature i.e. their physical properties are the same in all directions. 
4. When cut with a sharp edged tool, they cut into two pieces with irregular surfaces. 
5. They do not have definite heat of fusion 
6. They are called pseudo solids or super cooled liquids. This is because they have a tendency to flow, though very slowly.

Crystal lattice: A regular ordered arrangement of constituent particles in three dimensions is called crystal lattice.

Lattice points or lattice sites: The fixed positions on which the constituent particles are present are called lattice points or lattice sites. A group of lattice points which when repeated over and over again in 3 dimensions give the complete crystal lattice.

Unit cell: The unit cell is defined as the smallest repeating unit in space lattice which when repeated over and over again generates the complete crystal lattice. The crystal can consist of an infinite number of unit cells.

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