Allotropic Forms of Carbon - Part II

2. Structure of graphite  
Graphite itself comes in two allotropic forms. Naturally occurring graphite is called beta-graphite and it comes in a hexagonal form. Synthetically produced graphite is called alpha-graphite and it comes in a rhombohedral  form.

The hexagonal form of graphite has carbon atoms arranged in a hexagon; the hexagons form a plane. Each carbon atom in the hexagon is attached to only three other carbon atoms. The fourth electron of each carbon atom hangs loosely and is not bound. Graphite shows sp² hybridization. Within each layer, the carbon atoms are very strongly bonded by sigma () bonds. The bond angles are 120⁰ with each other. The links between adjacent layers is weak [1].

The sheet like structure with weak inter-layer forces makes graphite a very soft substance. The melting point of graphite is relatively high (3700⁰ C). This can be explained because of the strong overlap of the sigma () bonds within the carbon atoms in the planes. Sigma () bonds are known to be strong and they need a large amount of energy to break. Hence, just like diamond, graphite too has a high melting point.

Graphite is a good conductor of heat and electricity. This can be understood by looking at the loosely bound fourth electron of each carbon atom. The electron is relatively free to travel and move about in the crystal. The electrons are carriers of heat and electricity in a crystal. Hence it is clear why graphite is a good conductor of heat and electricity.

Density of graphite is 2.2 gm/cm³. From the structure of graphite it is clear that the distance between two hexagonal layers is large; thus the carbon atoms are not rigidly bound. This gives rise to low density of graphite as observed.

4. Structure of fullerenes  
Fullerenes are a molecular form of pure carbon discovered in 1985. They are cage-like structures of carbon atoms; the most abundant form produced is buckminsterfullerene (C₆₀), with 60 carbon atoms arranged in a spherical structure. There are larger fullerenes containing from 70 to 500 carbon atoms. Sometimes fullerenes are mistakenly called a "new form of carbon"; in fact, fullerenes have been found to exist in interstellar dust as well as in geological formations on Earth.

Fullerene cages are about 7-15 angstroms in diameter ( 1A° = 10^-10m). In atomic terms, their sizes are enormous. But fullerenes are still small compared to many organic molecules. Chemically, they are quite stable; breaking the balls requires temperatures of over 1000⁰ C (the exact number depends on which particular fullerene). At much lower temperatures (a few hundred degrees C) fullerenes will "sublime," which means vapor will form directly from the solid.

Pure C₆₀ is very interesting. Visually, it is quite different from both graphite and diamond -- it is a yellow powder, which turns pink when dissolved in certain solvents such as toluene. When exposed to strong ultraviolet light, the buckyballs polymerize, forming bonds between adjacent balls. In crystalline form C₆₀ is cubic (at each lattice point of a cube, there is a buckyball). Electrically, it is insulating. It shows electro-negativity and forms compounds easily with alkali atoms.

Summary  
We have seen in this chapter the various allotropic forms of carbon. The structure of diamond and graphite are so very different; the difference comes about due to their different bonding mechanisms. We have also seen a new form of carbon called fullerenes. Fullerenes have been recently discovered and have very interesting properties.

[1] The bonding between hexagonal layers is the best-known example of Van der Waal’s forces.

Next        Main        Previous