Allotropes of Carbon - Variability of Carbon

Variability of Carbon

The system of carbon allotropes spans an astounding range of extremes, considering that they are all merely structural formations of the same element.

Between diamond and graphite:

• Diamond crystallizes in the cubic system but graphite crystallizes in the hexagonal system.
• Diamond is clear and transparent, but graphite is black and opaque
• Diamond is the hardest mineral known (10 on the Mohs scale), but graphite is one of the softest (1–2 on Mohs scale).
• Diamond is the ultimate abrasive, but graphite is soft and is a very good lubricant.
• Diamond is an excellent electrical insulator, but graphite is a conductor of electricity.
• Diamond is an excellent thermal conductor, but some forms of graphite are used for thermal insulation (for example heat shields and firebreaks).
• At standard temperature and pressure, graphite is the thermodynamically stable form. Thus diamonds are not forever. The conversion from diamond to graphite, however, has a very high activation energy and is therefore extremely slow.

Despite the hardness of diamonds, the chemical bonds that hold the carbon atoms in diamonds together are actually weaker than those that hold together graphite. The difference is that in diamond, the bonds form an inflexible three-dimensional lattice. In graphite, the atoms are tightly bonded into sheets, but the sheets can slide easily over each other, making graphite soft.