AN INTRODUCTION TO :-
Carbon is without doubt one of the most versatile elements known to man, as can be seen by the fact that it is the basis of life on this planet. Carbon forms the basic building block of virtually all organic chemistry and of the 20 million known molecules, about 79% are classifies as organic. Carbon in its ground state has an electronic structure of 1s2 2s2 2p2 , but the 2s and 2p wavefunctions are normally hybridised to form 4 degenerate orbitals in a now sp3 hybridised atom. This allows the carbon atom to form 4 identical covalent bonds to other atoms and gives the atom a tetrahedral geometry. The reasons why carbon is such a diverse element is that it can form bonds to a huge range of other compounds, such as N, S, O, Cl, Br and P which crucially, are all thermodynamically stable. In addition to this, carbon can form single, double or triple bonds to other atoms and crucially, can also form these bonds to other carbon atoms. These carbon - carbon bonds have a very high intrinsic strength compared to similar bonds between other elements, for example, the bond strength of a C-C single bond has a value of 356kjmol-1 compared to a value of 226kjmol-1 for the equivalent Si-Si bond. As a result of this, it is possible to form carbon chains of phenomenal lengths, which is a property that allows materials such as carbon fibres to be produced. In addition to the wide range of organic molecules which contain carbon, there are several very important allotropes of carbon :-
As we all, know, diamond is a very strong and hard substance. This is a direct result of its microscopic structure, which comprises of each individual carbon atom being covalently bound to 4 other carbon atoms via C-C single bonds in a giant lattice structure :-
This provides an immensely strong structure and also gives rise to the other properties of diamond, which makes it such a desirable material.
To find out more about diamond, visit this Molecule of the Month page.
Graphite is almost the complete contrast to diamond, in that diamond is one of the worlds hardest, strongest materials, yet graphite is soft and brittle, whilst both are made entirely of carbon. The difference arises due to the way in which the individual carbon atoms are arranged within graphite's structure :-
Each individual carbon atom is sp2 hybridised (opposed to sp3 in diamond) and is bonded to 3 other carbon atoms via C-C bonds, each in a trigonal planar geometry, giving an overall hexagonal, "honeycomb" structure, like the one shown above. The remaining electron from each carbon atom is then delocalised within this structure. These free electrons explain why graphite is one of the few non - metallic structures which conducts electricity. The complete structure of graphite consists of many of these "sheets" lying one on top of the other, with weak inter - layer forces of attraction between each layer.
One interesting fact to point out is that graphite is the most stable allotrope of carbon, but is only 2.9kjmol-1 more stable than diamond at 300K and 1 atm. Consequently, it would be reasonable to assume that the inter conversion between the 2 would be relatively easy and that diamond would quickly decompose to graphite. In practice however, graphite has been converted directly to diamond, but only in extreme conditions to the magnitude of 3000K and 125kbar of pressure. Similarly, the decay of diamond to graphite has a half life of millions of years. The reasons for this can be explained using thermodynamics. Basically, a very large energy barrier exists between the 2 allotropes (the activation energy), which means that a lot of energy needs to be put into the system in order to inter convert between the two:-
At very high pressures, diamond actually becomes more stable than graphite energetically, which is why the inter conversion of graphite to diamond is possible.
FullerenesThe fullerenes are the third major allotrope of carbon. Fullerenes are large, spherical molecules, with a general formula of Cn, where n is generally greater than 60. The fullerenes (also known as "buckyballs") were discovered by the american engineer F. Buckminster Fuller. There has recently been a major research interest in the fullerenes and the possibility of using them in the treatment of a wide variety of diseases. The most widely recognised fullerine is the C60 form, which is comprised of 20 hexagonal faces and 12 pentagonal ones, which gives it the same shape as a football :-
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