Solution A
Diamond has such a poor conductivity that it is considered an electrical insulator. The reason for this is that each carbon forms 4 bonds with other carbons. When the band is produced we find that there is a fully occupied valence band and a conduction band, between which is a huge band gap. The band arises from the fact that diamond is a tightly bonded, stable covalent three dimensional crystal. This results in the insulator type of bands. A picture of the diamond structure is shown below.
In graphite, each carbon is bonded to three other carbons only, leaving a p orbital sticking out of the plane of the molecule that does not bond strongly to anything. This results in layers of carbon sitting above one another. The planes are held together by forces loosely termed as Van der Waals forces. The electrical conductivity in graphite arises from the fact that the unbonded p-orbitals overlap to form delocalized pi orbitals that allow for conduction to occur between the planes of carbon. This results in the band for graphite resembling that of a semi-metal.
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