PAGE6_CRUCICAL - 2. THE SOLID STATE Synopsis: INTRODUCTION:...

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1 Synopsis: INTRODUCTION: Solids are characterized by their high density and low compressibility as compared to those of the gas phase. The properties of solids indicate that the molecule (or ions) in them are relatively close together. Solids can be broadly classified into two categories, namely, crystalline and amorphous solids. The main characteristics of these are described in the following. Crystalline Solids: The outstanding characteristics of a crystal are its sharp melting point, its flat faces and sharp edges. These properties are due to a high degree of internal order which extends throughout the crystal (a definite pattern constantly repeating in space). This is know as long-range order. Amorphous Solids: Amorphous solids do not have the long-range order but have a short-range order. This characteristics may not be found around a similar atom placed at a distance from the other atom. Examples of amorphous solids are glass, used silica, rubber and polymers. Amorphous solids do not have the characteristics as possessed by crystalline solids. In many ways, they are more closely related to liquids than to the crystalline solids and are, therefore, regarded as supercooled liquids with high viscosity. A given material may be converted into the amorphous or glassy form by rapidly cooling the melt or freezing the vapour. CLASSIFICATION OF CRYSTALS BASED ON BOND TYPE The properties of most of the crystals are found to conform to one of the four general types of chemical bonds, in terms of which it is possible to classify them into four categories as described in the following. Molecular Crystals (or van der Waals Crystals): Molecular crystals are those in which the crystalline state is composed of an aggregate of discrete molecules held together by van der Waals forces. Because of these weaker forces, molecular crystals are soft and posses comparatively low melting points. Examples are CO 2 , CCl 4 , Ar and most of the organic compounds. Ionic Crystals: Ionic crystals involve electrostatic forces amongst their structural units. Because of stronger forces, ionic crystals are strong and likely to be brittle. The melting points are high, which decease with increasing size of the ions. In ionic crystals, some of the atoms may be held together by covalent bonds to form ions having definite positions and orientations in crystal lattice. Covalent Crystals: Covalent crystals involve forces of chemical nature (covalent bonds) extended in three dimensions. These forces are strong, and consequently the crystals are strong and hard, with high melting points. Examples are diamond, silicon, etc. Metallic crystals: Electrons are held loosely in these type of crystals. They are god conductors or electricity.
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This note was uploaded on 04/05/2011 for the course ENGIN 45 taught by Professor Devine during the Spring '07 term at University of California, Berkeley.

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PAGE6_CRUCICAL - 2. THE SOLID STATE Synopsis: INTRODUCTION:...

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