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:

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

AMORPHOUS AND CRYSTALLINE SOLIDS :

Solids can be classified as crystalline or amorphous on the basis of the
nature of order present in the arrangement of their constituent particles.
A crystalline solid usually consists of a large number of small crystals,
each of them having a definite characteristic geometrical shape. In a crystal,
the arrangement of constituent particles (atoms, molecules or ions) is
ordered. It has long range order which means that there is a regular pattern
of arrangement of particles which repeats itself periodically over the entire
crystal. Sodium chloride and quartz are typical examples of crystalline
solids. An amorphous solid (Greek amorphos = no form) consists of particles
of irregular shape. The arrangement of constituent particles (atoms,
molecules or ions) in such a solid has only short range order. In such an
arrangement, a regular and periodically repeating pattern is observed over
short distances only. Such portions are scattered and in between the
arrangement is disordered. The structures of quartz (crystalline) and
quartz glass (amorphous) While the two structures are almost identical, yet in the case of amorphous quartz glass there is no long range order. The structure of amorphous solids is similar to that of liquids. Glass, rubber and plastics are typical examples of amorphous solids. Due to the differences in the arrangement of the constituent particles, the two types of solids differ
in their properties.

Crystalline solids have a sharp melting point. On the other hand, amorphous solids soften over a range of temperature and can be moulded and blown into various shapes. On heating they become crystalline at some temperature. Some glass objects from ancient civilisations are found to become milky in
appearance because of some crystallisation. Like liquids, amorphous solids have a tendency to flow though very slowly. Therefore, sometimes these are
called pseudo solids or super cooled liquids. Glass panes fixed to windows or doors of old building are invariably found to be slightly thicker at the
bottom than at the top. This is because the glass flows down very slowly and makes the bottom portion slightly thicker.

Crystalline solids are anisotropic in nature, that is, some of their physical properties like electrical resistance or refractive index show different values
when measured along different directions in the same crystals. This arises from different arrangement of particles in different directions. Since the arrangement of particles is different along different directions, the value of same physical
property is found to be different along each direction.

Amorphous solids on the other hand are isotropic in nature. It is because there is no long range order in them and arrangement is irregular along all the directions. Therefore, value of any physical property would be same along any direction.

Amorphous solids are useful materials. Glass, rubber and plastics
find many applications in our daily lives. Amorphous silicon is one of the
best photovoltaic material available for conversion of sunlight into electricity.