The many divisions and sub-divisions which have appeared in science over the past century apparently present a bewildering confusion. Thus, the many branches of science are chemistry, physics, botany, zoology, geology, meteorology, agronomy and several others. Each of these is further divided into subfields. Chemistry, for instance, has branched out into physical chemistry, organic chemistry, inorganic chemistry, biochemistry, analytical chemistry, polymer chemistry, and so on. In the broad field of physics, again, we have sub-divisions of heat, light, sound, magnetism, electricity, and several others. However, behind the apparent diversity of these fields and specializations, there is a unifying theme running through all these branches. The atomic and molecular theory of matter which indeed is the starting point for each of these sciences provides this bond. All matter is made up of small particles called atoms and molecules and it is through detailed understanding of their structures and interactions that we can understand the complex behaviour of matter and materials around us.
Ordinary matter is an aggregate of atoms or of molecules. A molecule, in turn, may be defined as a grouping of a limited number of atoms which are strongly bonded together but whose bonds with the atoms of other adjacent molecules are relatively weak. These groups of atoms therefore act as a unit in the constitution of matter. Forces of attraction or bonds which are responsible for the aggregation of atoms and molecules into ordinary matter can be divided into primary bonds or chemical bonds, and secondary bonds or intermolecular attraction forces, frequently called van der Waals forces. Primary bonds are of three general types: (1) ionic or electrostatic bonds, (2) covalent, atomic or homopolar bonds, and (3) metallic bonds. This classification of primary bonds is, however, not a rigorous one. Thus, there are many bonds which do not strictly belong to any of the three types but are intermediate between truly ionic and truly covalent bonds, between ionic and metallic bonds, or between covalent and metallic bonds.
In the previous chapter we studied the major types of bonding in solids (ionic, covalent, and metallic), and now we shall see how the character of a phase depends on the nature of distribution of the component atoms in it. The phase may be solid, liquid or gaseous, which are the three commonly occurring states of matter. Although solid phases are of primary importance in materials science, gases and liquids are also of interest in their own right.
A crystal is a regular three-dimensional design and is a consequence of the regular arrangement of the atoms, ions, or molecules of which it is built up. However, most of the solid matter we usually come across hardly shows outward evidence of crystalline form; and this is because it is polycrystalline, composed of tiny crystals having random orientations with respect to each other. Metals are polycrystalline and so also are most minerals. Even the very finely divided precipitates and various carbons, formerly regarded as amorphous, are in fact composed of exceedingly fine crystals. Naturally, the term ‘solid’ is often taken as synonymous with crystalline.
Although we speak of an ideal crystal as consisting of an extended array of atoms with perfectly regular periodicity in all directions, such perfection is rarely found in the internal structure of real crystals. This is not surprising considering the enormous magnitude of activity that occurs on an atomic scale on the surface of a crystal during growth. Thus, even for a very small growth rate, such as 1 mm per day, about one hundred layers of atoms are deposited on the surface per second. On an atomic scale, the presence of imperfections or defects is thus very common. Moreover, it may be shown by thermodynamic reasoning that crystal defects are to be expected at all temperatures above absolute zero, although the fraction of defects at ordinary temperatures may be negligibly small. Defects in crystals may be of several forms—point defects, line defects and planar defects. Discussion of these defects will occupy most of this chapter.
