CRYSTAL STRUCTURE

CRYSTAL STRUCTURE

 Properties of an individual atom are determined by its atomic structure. In this respect, valence electrons play an important role in producing most of its chemical, electrical and optical properties. These atoms combine toge­ther to form crystals. The arrangement of atoms in the interior of a crystal is called crystal structure. This structure is determined by: (a) grouping of the atoms, (b) bonding between them, (c) type of the space lattice formed, (d) parameters of the unit cell, and (e) the number and position of atoms per unit cell. Let us try to understand these factors.

  Grouping of Atoms: Metals are aggregate of atoms. Metallic properties depend, not only on the nature of the atoms but also on the manner in which atoms have been assembled. Depending upon the type of their grouping, materials can be classified into three categories, namely: (i) molecular structures, (ii) crystal struc­tures and (iii) amorphous structures.

(i) Molecular structures: These structures are formed when a limited number of atoms come together and get strongly bonded to one another. The resulting groups are called molecules, for example, H₂O, C0₂, CCl₄, 02, N2, etc. Within these molecules, the atoms are held together by strong attractive forces that usually have covalent or ionic bonds. Similar group of atoms have rela­tively weak bonds among themselves. The groups have no net charge.

(ii) Crystal structures: Atomic arrangements which have a repetitive pattern in all the three dimensions of space are called crystal structures or crystals. In such struc­tures, a fundamental unit of the arrangement repeats itself at regular inter­vals in three dimensions, throughout the interior of the crystal. Most of the metals are crystalline and consist of crystals.

(iii) Amorphous structures are formed when atoms do not have the long range repetitive pattern of arrangement and the pattern breaks at different places. Common examples of this group are glasses and polymers. Most glasses consist primarily of silicate ions, SiO₂, to which an appreciable number of large-sized atoms such as sodium have been added. The added sodium and other atoms, since they do not fit into the silicate structure very well, make it more difficult for crystallization to occur when the melt is cooled. Glass is thus a supercooled liquid having a very high viscosity.

Structures of polymers greatly differ from those of metals. Metals and ceramics are aggregates of atoms which can be regarded as arrays of hard and spherical atoms in three directions of space, while the structure of poly­mers is composed of molecules of extremely high molecular weight which cannot be regarded as hard spheres. Most polymers have the form of long, flexible chains in which molecules are twisted up and inter-twined^- with each other. A typical molecule of polyethylene, for example, might be represented by a chain with a length of about 5 X 10^-2 micron and a dia­meter of less than 5 X 10^-6 micron. A molecule such as this, in a mass of material, can readily become knotted and entangled with the surrounding molecules.

  Binding in Solids: Since most of the metals are solid at room temperature, we shall consider only solids. Even in solid state, materials can have a very wide range of properties. This is partly due to the fact that atoms have different types of bindings among themselves. All the atoms of a crystal have definite types of attractive forces among themselves which keep the atoms bonded toge­ther. The attractive forces could be of the following types.

(a) Metallic Bond: This type of bond results when each atom of the metal contributes its valence electrons to the formation of an electron cloud that spreads throughout the solid metal. A characteristic of the metallic bond is that the conduction of electricity and heat are produced by the free move­ment of valence electrons through the metal. All metal conductors show this type of bond. The resistance to the free movement of the electron occurs     if it collides with other electrons. This collision of the electrons interferes with the flow of the electrons and accounts for the resistivity in metals. Metallic crystals are malleable and have variable hardness and melting point.

(b) Ionic Bond: This bond exists between two unlike atoms. If an electron is transferred from a metallic atom to a non-metallic atom, the two result­ing ions are held together by electrostatic attraction. Examples are the sodium and chlorine atoms. Sodium atom gives away its valence electron and becomes a positive ion, while chlorine atom takes the electron to fill its last orbit and becomes a negative ion. Ionic crystals have poor electrical conductivity, high hardness and high melting point.

(c) Covalent Bond: This bond is formed by sharing of electrons between adjacent atoms. An excellent example of covalent bonding is found in the chlorine molecule. Here, the outer shell of each atom possesses seven elec­trons. Each chlorine atom would like to gain an electron, and thus form a stable octet. This can be done by sharing of two electrons between pairs of chlorine atoms. Each atom contributes one electron for the sharing process. The diamond form of carbon is an another example of this bond where four valence electrons are shared between four neighbouring atoms. Other examples of this type of bond are hydrogen, nitrogen etc. Covalent crystals are characterized by poor electrical conductivity and high hardness.

(d) Van der Waals Bond: Inert gases and molecules like methane, which have no valence electrons available for crystalline binding, obtain a weak attractive force as a result of polarization of electrical charges. Polarization is displacement of the centres of positive and negative charges in an electri­cally neutral atom or molecule when as it is brought close to its neighbouring atom. Its neighbours also become polarized. The resulting weak electrical attraction between neighbouring atoms or molecules is the Van der Waals force. This force can be overcome by the disrupting effect of thermal mo­tion of atoms and molecules at higher temperatures.

Members of the halogen family-fluorine, chlorine, bromine and iodine all form stable diatomic molecules with covalent bonds. The additional forces whⁱch hold the molecules together are of the Van der Waals type. Elements possessing Van der Waals bond are soft, have poor electrical conductivity and low melting point.

Schematic representation of the various types of bonds in solids is shown in Figure 2.1.

Space lattice and unit cell: A crystalline substance is one which is made of crystals or parts of crystals. In a crystal, the atoms are arranged in a periodic and regular geometric pattern in space. The arrangement of atoms in a crystal can be described with respect to a three-dimensional net of straight lines, called a space lattice, as shown in Figure 2.2. The intersections of the lines are points of a space lattice. These points may be occupied by the atoms in crystals or they may be the points about which several atoms are clustered. The impor­tant characteristic of a space lattice is that every point has identical sur­roundings.