Difference between Crystalline and Amorphous Solids

Difference between Crystalline and Amorphous Solids.

 Crystalline and amorphous solids differ from one another in the following respects

1.  Characteristic geometry

A crystalline solid has a definite and regular geometry due to definite and orderly arrangement of molecules or atoms in three-dimensional space. An amorphous solid, on the other hand, does not have any pattern of arrangement of molecules or atoms and, therefore, does not have any define geometrical shape. It has been found that even if some orderly arrangement of molecules or atoms exists in a few amorphous solids, it does not extend more than a few Angstrom units. Thus unlike crystalline solids, amorphous solids do not have a long range order.

2.  Melting points

As a solid is heated, it's molecular vibrations increase and ultimately becomes so great that molecules break away from their fixed positions. They now begin to move more freely and have rotational motion as well. The solid now changes into liquid state. The temperature at which this occurs is known as the melting point.

A crystalline substance has a sharp melting point, i.e., it changes abruptly into liquid state. An amorphous substance, on the contrary, does not have a sharp melting point. For example, if glass is heated gradually, it softens and starts to flow without undergoing a definite and abrupt change into liquid state. The amorphous solids are, therefore, regarded as liquids at all temperatures. There is some justification for this view because it is known form X-ray examination that amorphous substance do not have well-ordered molecular or atomic arrangements. Strictly speaking, solid state refers to crystalline state, i.e., only a crystalline material can be considered to be a true solid.

3.  Isotropy and Anisotropy

Amorphous substances differ from crystalline solids and resemble liquids in another important respect. The properties such as electrical conductivity thermal conductivity, mechanical strength and refractive index are the same in all directions. Amorphous substances are, therefore, said to be isotropic. Liquids and gases are also isotropic. Crystalline solids, on the other hand, are anisotropic, i.e., their physical properties are different in different directions. For example, the velocity of light passing through a crystal varies with the direction in which it is measured. Thus, a ray of light entering such a crystal may split up into two components each following a different path and travelling with a different velocity. This phenomenon is known as double refraction. Thus, anisotropy in itself is a strong evidence for the existence of ordered molecular arrangements in such materials. This can be shown on reference to Fig. in which a simple two-dimensional arrangement of only two different kinds of atoms is depicted.

If the properties are measured along the direction indicated by the slanting line CD, they will be different from those measured in the direction indicated by the vertical line AB. The reason is that while in the first case, each row is made up of alternate type of atoms, in the second case, each row is made up of one type of atoms only. In amorphous solids as well as in liquids and gases, atoms or molecules are arranged at random and in a disorderly manner and, therefore, all directions are identical and all properties are alike in all directions.