Allotropic Forms of Carbon - Part I

Carbon (in Latin carbo means charcoal) is an element of prehistoric discovery. It is very widely distributed in nature. It is found in abundance in the sun, stars, comets, and atmospheres of most planets. Carbon in the form of microscopic diamonds is found in some meteorites.

Elements can either be in structural crystalline forms or may occur in a structure-less amorphous form. Some elements like carbon, sulphur, tin, oxygen, etc. are found in more than one structural forms. That is some elements can have several different structural forms while in the same physical state. These differing forms are called allotropes. For example in crystalline form, pure carbon is found as graphite, diamond, Buckministerfullerene. 

There is another form of crystalline carbon known as “white” carbon, but not much is known about it. Conditions such as temperature and pressure determine which allotrope of carbon occurs. A diamond is hard, clear, and shiny. It is formed under very high temperatures and pressure. Graphite is black, with a slippery, greasy feel. Graphite is formed on heating coke or coal. Buckministerfullerenes[1] contain 60 carbon atoms arranged in a round molecule resembling a soccer ball.  

It is interesting to note the differences between diamond and graphite. 

• Diamond is the hardest substance known in nature. Graphite is soft and slippery to touch.  

• Diamond is a transparent substance whereas graphite is black and opaque. 

• Diamond is a poor conductor of electricity, but is a good conductor of heat. Graphite on the other hand is a good conductor of  heat and electricity.

What we will study in this chapter : 

1. Structure of diamond 
2. Structure of graphite 
3. Structure of fullerenes  

1. Structure of diamond  
Each molecule of diamond is quite large and the crystalline structure in which the molecules arrange themselves is cubic. Each carbon atom is covalently bonded with four other carbon atoms. These four atoms are again strongly bonded with other four carbon atoms. All the bonds are sp³ hybridized. The sigma () bonds formed, have maximum electronic overlap with each other. The bond angles are at 109^o28’ with each other. The structure formed is therefore very rigid. This makes diamond one of the hardest substances that are found naturally.

All the physical properties of diamond come about due to its strong bonding. High amount of heat has to be introduced to break the bonds; hence diamond shows very high melting point (about 3500°C). Since the sigma () bonds are saturated or are overlapping to a maximum degree with the next carbon atom’s electron, the diamond crystal is compact giving a high density of about 3.5 gm/cm³.

By looking at the sigma () bonds again, we can understand why diamond is a bad conductor of electricity. Electricity is conducted in a crystal by electrons that are relatively free. Electricity is conducted by electrons which can hop from one site to the other. In a sigma () bond, the electrons are completely shared by neighboring carbon atoms and are not “free” to go from site to site conducting  electricity. Diamond is a good conductor of heat because the electrons in the bonds are shared and hence the heat can be dissipated easily.

[1] Buckminsterfullerene is a carbon allotrope with 60 carbon atoms, named after Buckminster Fuller, the man who designed the geodesic dome. It shows a great potential for use in many fields. It's discoverers recently earned the 1996 Nobel Prize for Chemistry. 

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