the solid state

INTRODUCTION :

We are mostly surrounded by solids and we use them more often than liquids and gases. For different applications we need solids with widely different
properties. These properties depend upon the nature of constituent particles and the binding forces operating between them. Therefore, study of the structure of solids is important. The correlation between structure and properties helps in discovering new solid materials with desired properties like high temperature
superconductors, magnetic materials, biodegradable polymers for packaging, biocompliant solids for surgical implants, etc.

From our earlier studies, we know that liquids and gases are called fluids because of their ability to flow. The fluidity in both of these states is due to the fact that the molecules are free to move about. On the contrary, the constituent particles in solids have fixed positions and can only oscillate about their mean positions. This explains the rigidity in solids. In crystalline solids, the constituent particles are arranged in regular patterns.

In this Unit, we shall discuss different possible arrangements of particles resulting in several types of structures. The correlation between the nature of
interactions within the constituent particles and several properties of solids will also be explored. How these properties get modified due to the structural
imperfections or by the presence of impurities in minute amounts would also be discussed.

general characteristics of solid state :

you have learnt that matter can exist in three states namely, solid, liquid and gas. Under a given set of conditions of temperature and pressure, which of these would be the most stable state of a given substance depends upon the net effect of two opposing factors. Intermolecular forces tend to keep the molecules (or atoms or ions) closer, whereas thermal energy tends to keep them apart by making them move faster. At sufficiently low temperature, the thermal energy is low and intermolecular forces bring them so close that they cling to one another
and occupy fixed positions. These can still oscillate about their mean positions and the substance exists in solid state. The following are the characteristic properties of the solid state:

1. They have definite mass, volume and shape.
2. Intermolecular distances are short.
3.  Intermolecular forces are strong.
4.  Their constituent particles (atoms, molecules or ions) have fixed
   positions and can only oscillate about their mean positions.
5.  They are incompressible and rigid.

AMORPHOUS AND CRYSTALLINE SOLIDS :