The above properties suggest that the properties of the solids not only depend upon the nature of the constituents, but also on their arrangements.
Based on their structural features solids can be classified into three categories:
Crystalline solids
Amorphous solids
Polycrystalline solids
Crystalline Solids
The substances whose constituents are arranged in definite orderly arrangements are called crystalline solids. Many naturally occurring solid substances occur in the crystalline form. Some common examples of crystalline solids are sodium chloride, sulphur, diamond, sugar, etc.
They have the following characteristics:
They posses characteristic geometric shapes. The constituent particles in crystals are usually held by strong inter atomic, inter-ionic or intermolecular forces. Particles vibrate about their equilibrium positions and do not posses translational motion.
Crystalline solids have sharp melting points, indicating the presence of a long range order arrangement in them. The long-range order is due to the regular arrangement of the constituents (molecules, atoms or ions) throughout the three dimensional network of crystals. For example, experiments (X-ray diffraction method) show that in a crystal of sodium chloride, the constituents Na+ and Cl- ions are present at alternate sites as shown below:
Na+ Cl- Na+ Cl- Na+ Cl-
Cl- Na+ Cl- Na+ Cl- Na+
Na+ Cl- Na+ Cl- Na+ Cl-
Cl- Na+ Cl- Na+ Cl- Na+
Though shown in two dimensions this systematic long-range order is also found in three dimensions, with each Na+ surrounded by six ions and vice versa. This order is due to strong coulombic forces of attraction between Na+ and Cl- ions. Similar regular arrangements are found in other solids too.
Crystalline solids are anisotropic by nature i.e., their mechanical, electrical and optical properties depend upon the direction along which they are measured.
When cut or hammered gently they show a clean fracture along a smooth surface.
Amorphous Solids
Substances whose constituents are not arranged in an orderly manner are called amorphous solids. They are also called pseudo solids and differ from crystalline solids in many respects. The common examples of amorphous solids are glass, rubber, fused silica, plastics, etc.
They have the following characteristics:
They do not occur in characteristic geometric shapes. They posses properties of incompressibility and rigidity to some extent.
Their mechanical, electrical and optical properties do not depend upon the direction along which they are measured. They are isotropic and in this respect resemble liquids and are sometimes referred to as super cooled liquids.
Amorphous solids do not have sharp melting points. They have some orderly arrangement but it is not extended to more than a few Angstrom units. Thus, amorphous solids are said to have short-range order. On heating they melt and begin to flow like liquids.
When cut or hammered they break in an irregular manner.
Polycrystalline Solids
These are crystalline solids, constituted of very fine crystals, not seen by the naked eye. These solids appear amorphous but are not so because the fine crystals are randomly oriented. Thus even though each individual crystal is anisotropic the polycrystalline materials as a whole appear to be isotropic. Metal powders are polycrystalline in nature.
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