Some solids, like sodium chloride, sulphur and sugar, besides being incompressible and rigid, have also characteristic geometrical forms. In these solids the atoms or molecules are arranged in a very regular and orderly fashion in a three dimensional pattern. Such substances are called crystalline solid.
The X-ray diffraction studies reveal that their ultimate particles (viz., molecules, atoms or ions) are arranged in a definite pattern throughout the entire three-dimensional net-work of a crystal. This definite and ordered arrangement of molecules, atoms or ions (as the case may be) extends over a large distance. This is termed as long-range order.
The outstanding characteristics of a crystalline solid are its sharp melting point. Crystalline solids are anisotropic since they exhibit different physical properties in all directions e.g., the electrical and thermal conductivities are different in different directions.
There is another category of solids such as glass, rubber and plastics, which possess properties of incompressibility and rigidity to a certain extent but do not have definite geometrical forms. Such substances are called amorphous solids
Amorphous solids (from the Greek words for 'with out form') neither have ordered arrangement nor sharp melting point like crystals but when heated, they become pliable until they assume the properties usually related to liquids. These solids lack well-defined faces and shapes. Many amorphous solids are mixture of molecules that do not stick together well. Most others are composed of large complicated molecules. Amorphous solids are therefore regarded as super cooled liquids with high material becomes rigid but there the forces of attraction holding the molecules together are so great that the material becomes rigid but there is no regularity of structure. Thus, amorphous solids do not melt at specific temperatures. Instead they soften over a temperature range as intermolecular forces of various strengths are overcome.
Amorphous solids are isotropic as they exhibit same physical properties in all the directions.
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.
Size and shape of crystals
Several naturally occurring solids have definite crystalline shapes, which can be recognized easily. There are many other solid materials, which occur as powders or agglomerates of fine particles and appear to be amorphous. But when an individual particle is examined under a microscope, it is also seen to have a definite crystalline shape. Such solids, in which the crystals are so small that can be recognized only under a powerful microscope, are said to be microcrystalline. The size of a crystal depends on the rate at which it is formed: the slower the rate the bigger the crystal. This is because time is needed by the atoms or molecules to find their proper positions in the crystal structure. Thus, large transparent crystals of sodium chloride, silver chloride, lithium chloride, etc., can be prepared by melting these salts and allowing them to cool very slowly at a uniform rate. It is for this reason that crystals of most of the minerals formed by geological processes are often very large.
Crystal possess the following characteristic feature:
i)Faces: Crystals are bound by plane faces. The surfaces usually plannar and arranged on a definite plane (as a result of internal geometry), which bind crystals are called faces.
Faces are of two types:
Like: A crystal having all faces alike e.g. Fluorspar.
Unlike: A crystal having all faces not alike e.g. Galena.
ii) Form: All the faces corresponding to a crystal are said to constitute a form.
iii) Edges: The intersection of two adjacent faces gives rise to the formation of edge.
iv) Interfacial Angle: The angle between the normals to the two intersecting faces is called interfacial angle.
Although the size of the faces or even faces of the crystals of the same substance may vary widely with conditions of formation, etc., yet the interfacial angles for any two corresponding faces of the crystals remain invariably the same throughout.
Although the external shape is different yet the interfacial angles are same. The measurement of interfacial angles in crystals is, therefore, important in the study of crystals. The subject is known as crystallography.