Carbon exist two allotropic forms are Diamond and Graphite, Discussing about Structure of Diamond and Graphite we must know There are two common allotrops of carbon, Crystalline forms of elemental carbon are different. Besides these, there are microcrystalline forms which were previously regarded as amorphous. These are coal, coke, charcoal, carbon black, etc. Silicon is known to exist in crystalline and amorphous forms. Germanium also exists in two crystalline form Likewise, tin exists in three solid forms: grey tin, white tin and rhombic tin. Grey tin is stable below 18°C white tin above 18°C and rhombic tin above 161°C. Grey tin exists usually in powder form and a poor conductor of electricity.
therefore, lacks metallic properties. Its density is 5-75 g cm³. White tin, on the other hand, is soft, ductile and a good conductor of electricity. It has, therefore, metallic character. It has also the usual metallic lustre. Being stable at room temperature, it is the most common form of tin. Its density is 7-30 g cm
Structure of Diamond
Diamond is effectively on electrical insulator and it is the hardest no natural substance and here the ultimate abrasive Diamond is one of the most highly priced gemstone, this is the physical significance of Diamond. Diamond are typically colourless through diamonds (used for making drills. Black Mont naturally occurring industrial. Or as one abrasive powders) are of ton diamond contains a trace of Nitrogen but blue diamond contains a trace of Al instead. In Diamond each carbon atom is tetrahedrally surrounded by four other carbon atom each at a distance of 1.5.A°.’ The tetrahedra is linked together in a three dimensional (30) giant molecule The unit cell is cubic and strong covalent bonds extend in all direction so the melting print abnormally high (about 39.30°c) and the structure may be hard
It will be seen that each carbon atom due to linked to four surrounding carbon atoms lying at the corners of the tetrahedron by strong covalent bonds through sharing of their valency electrons. The result is a very big, three-dimensional polymer. Due to high strength of covalent bonds holding numerous carbon atoms together, diamond is very hard and has high density. Since melting of diamond requires breaking of the strong covalent bonds, the melting point of diamond is unusually high, being close to 3500°C. Diamond is the purest form of carbon and is the hardest substance known. It has a very high refractive index (2-417) and a very high density (3-51 g cm-3 at 25°C). It is transparent to X-rays and is a bad conductor of heat and electricity.
Structure of Graphite
Graphite is composed of flat two two dimensional sheet of carbon atoms. Each sheet Hexagonal net of Carbon atoms and may be regarded a few system of benzene ring. The layer are held together by relatively with Vander walls forces. In alpha (a) graphite the layer are arises in sequence ABAB…. Twith third layer exactly above the first layer. In Bita graphite (B) the order of the layer is ABCABC…. At the two forms are interconvertable. On heating Graphite turns into B and grinding turns & into B. in both form the c-c bond length with in a sheet are 1.41 A° and the distance between two layers is 3.35A°. So the bonding between layers is weak. Graphite cleaves easily between the layers which accounts for the remarkable softness of crystal
The distance between the two successive layers is large so the possibility of covalent bonding between carbon atoms in different layers. Hence, the fourth valency of each carbon atom remains unsatisfied, ie,, the fourth valency electron remains unpaired or free. This free electron can easily move from one carbon atom to another under the influence of applied potential. This permits the passage of electricity through graphite and makes graphite a good conductor of electricity.
Structure of Fullerene
Fullerenes are allotropes of carbon having the general formula C2 (n=30-48). These are prepared by passing high voltage current through graphite rods in inert atmosphere of helium. The graphite the rods sublimes producing a fluffy mass called ‘fullerene soot. This soot is then dissolved in organic solvents such as benzene and chromatographed when the solutions of pure C60 (about 79%) and (about 20%) separate out as major fractions. Minor amounts (about 1%) of higher fullerenes of C60, C70, C80, etc., also separate out in the process. The solutions when concentrated, yield crystals of fullerenes containing solvent molecules trapped in the interstices. The sublimation of these cry in vacuum yields solvent-free fullerene crystals.
The most common of these Fullerenes, C60, contains 20 hexagons and 12 pentagons of carbon a fused with one another resulting in a spherical football-like geometry, as shown.
There are two types of C-C bonds in the structure of C60 one of which is 1-48 A° and the other is 1-38 A° in length. Although the fullerene C60 is one of the most strained molecules known so far yet it shows high kinetic stability. Nevertheless, its thermal stability is less than that of graphite and diamond. It starts decomposing at about 700°C.
In the fullerene C60 there are eight distinct types of C-C bonds with lengths varying between 1:39 A and 1-54A° This fullerene has also spherical, football-like structure containing several hexagons and pentagons of carbon atoms fused together. Higher fullerenes have similar football-like structures.