Solid State chemistry

Solids can be classified on the basis of nature of order in the arrangement of constituent particles (atoms, ions or molecules) into two types as 

1. Crystalline solids

2. Amorphous solids

1. Crystalline solids (Topic for CSIR-NET Coaching in Chemistry)

The substance whose constituent particles (atoms, ions or molecules) are arranged in a defined geometric pattern in three dimensional space are called crystalline solids. X-ray diffraction studies of these crystalline solids reveal that in these solids, the constituents (atoms, ions or molecules) are arranged in a definite orderly arrangement. The regular arrangement of the constituents extends throughout the three dimensional network of crystals. In other words, the crystalline substance are said to have Long range order. This means that in crystalline solids, there is a regular pattern or arrangement of particles which repeats itself periodically aver the entire crystal. In fact a crystalline solid consists of a large number of small crystals, each of them having a definite characteristic geometrical shape. Most of the substances (element and compounds) from crystalline solids. For example, The element like copper, silver, iron, sulphur, phosphorous and iodine and common compounds such as sodium, chloride, potassium nitrate, zine sulphide, quartz, etc., all form crystalline solids 

2. Amorphous solids

The substance whose constituent particles are not arranged in any regular arrangement are called amorphous solids (Greek amorphous meaning no form). The amorphous solids have a random, disordered arrangement of constituent. Therefore, they do not have definite shape or form like crystalline solids. These substances also possess the properties of incompressibility and rigidity to certain extent. It is observed that some amorphous solids have some orderly arrangement but it is not extended to more than a few Angstrom units. Tus, t6he amorphous solids, a regular and periodically repeating pattern is observed over short distance only. These regular patterns are scattered and in between, the arrangement is disordered. The common examples of amorphous solids are glass, rubber, plastic, etc.

Differences between Crystalline and Amorphous solids

Due to differences in the arrangement of constituent particles. The crystalline and amorphous solids differ in their properties. The importance differences between Crystalline and Amorphous solids are;

1. Characteristic geometry- The crystalline solids have definite regular geometry because of orderly arrangement of constituents (atom, molecules or ions) in three dimensional space. On the other hand, amorphous solids do not Possess any regular arrangement of constituent particles and therefore, do not have any definite geometric shapes.

2. Melting points. The crystalline solids have sharp melting points i.e. they change abruptly into liquid state at a fixed temperature. On the other hand amorphous solids do not have sharp melting point. Rather, on heating they first soften and then melt over a range of temperature. This property is made use of in molding and blowing them into various shapes. For example, when glass is heated gradually, it softens and start to flow without undergoing a definite and abrupt change into liquid state. Therefore, glass can be moulded and give any desired shape.it has been observed that on heating and then cooling slowly (i.e. Annealing) amorphous solids become crystalline at some temperature. It is for this reason that some glass objects from ancient civilizations are founds to become milky in appearance because of some crystallization having taken place.

3. Isotropy and anisotropy. In crystalline substances, some of the physical properties such as electrical conductivity, thermal conductivity, mechanical strength, refractive index, etc, have different values when measured along different directions in the same crystal. Such substance which have physical properties different in different directions are called anisotropic Thus crystalline substances are anisotropic. On the other hand, amorphous substances have physical properties same in all directions and are called isotropic liquids and gases are also isotropic. For example, the velocity of light passing through a crystal varies with the direction in which it is measures. Thus anisotropy is a strong evidence for the existence of orderly arrangement in crystalline solids.

4. Cleavage. Crystalline solids can be cleaved along definite planes. When cut with a sharp edged tool (e.g. Knife) they split into two pieces and they newly generated surface are plain and smooth. On the other hand, amorphous solids cannot be cleaved along definite planes. When cut with a sharps edged tool (e.g. Knife) they undergo irregular breakage and split into two pieces with irregular surface 

Uses of Amorphous Solids

Amorphous solids such as glass, rubber, plastic, etc, find many uses in our daily lives because of their unique characteristics for example.

(i) The most widely used amorphous solid are inorganic glasses which are used in construction, housewares, laboratory ware, etc.

(ii) Amorphous silicon is the best materials for converting the sunlight into electricity (in photovoltaic cells).

(iii) Rubber is also an amorphous solid which is used in making tyres, shoes, soles, etc.

(iv) A large number of plastics which are amorphous solids are used in making articles of daily use.

The above mentioned topic is essential for CSIR-NET  Coaching in Chemistry. It is also a hot topic for Chemistry JAM Coaching