Greetings,
This post continues a discussion on network solids, started in the last post. Two allotropes of carbon are explored.
Carbon is a very versatile element in that it can bond with itself & form single,double, and triple bonds. The result is a wide variety of structural geometries. Even carbon as a pure element exists in four distinct forms; diamond, graphite, Buckminsterfullerene, and nanoTubes. Diamond consists of an array of tetrahedral structures which join together to produce a very strong crystal. The other three forms are variations of carbon's ability to form fused ring sheets. In the case of graphite, these sheets are flat and loosely connected via sigma-bonds of p-atomic orbitals. Sigma bonds are an end-to-end orbital overlap as opposed to pi-bonds which involve a side-to-side orbital overlap. The following graphic shows the incredibly different structures of carbon as diamond and as graphite.
The overall geometry of the diamond crystal results from a combination of very many unit cells. The geometry of the unit cell itself arises from a combination of four distinct tetrahedral structures.
The vastly different geometry of graphite is immediately obvious in the diagram. Very many individual fused ring sheets exist. The bonding within each sheet is very strong but weak between the sheets. As a result, chunks of fused ring sheets are easily "torn off" from the network structure. This manifests itself as the flat-faced flaked appearance of broken pencil lead.
That's all for now. Thank you for reading!
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