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. Therefore 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 ficilitate electrial conduction. Thus diamond is an excellent electrical insulator.

 Courtesy : The Hindu