Why is diamond an electrical insulator while it is a good conductor of heat?
Diamond is a good thermal conductor. But it is an excellent
electrical insulator. How?
Diamond crystal is a three-dimensional network of carbon atoms.
All carbon atoms in the network are strongly bonded by
carbon-carbon covalent bonds.
The diamond crystal has a highly symmetric cubic structure. The
carbon atoms in diamond are precisely aligned. Thus diamond is an
ideal crystal. Atoms in the crystal lattices in solids vibrate.
These vibrations, called the atomic vibrations facilitate
thermal conduction (transport of heat) in solids. In an ideal
crystal, the lattices are so precisely aligned that they do not
interact with each other.
Therefore an ideal crystal conducts better than a non-ideal
crystal resulting in ideal crystals having good thermal
conductivity, which is a measure of heat conduction. Diamond being
an ideal crystal is thus a good thermal conductor.
Mobile electrons facilitate electrical conduction - flow of
current in solids. There are no free mobile electrons in diamond
crystal to facilitate electrial conduction. Thus diamond is an
excellent electrical insulator. In more simple terms, Diamond has a
unique atomic structure. Each of the Carbon atom's valence
electrons is taken up in covalent bonding, so each electron is
localised. This means that no one electron is free to pass
an electric current through the Diamond - thus, Diamond cannot
carry an electrical current; there is nothing to pass the current