Allotropes of carbon

Diamond, graphite and fullerenes (substances that include nanotubes and ‘buckyballs’, such as buckminsterfullerene) are three allotropes of pure carbon.

In all three allotropes, the carbon atoms are joined by strong covalent bonds, but in such different arrangements that the properties of the allotropes are very different.

Diamond

A diamond is one giant molecule of carbon atoms. Diamonds are colourless and transparent. They sparkle and reflect light, which is why they are described as lustrous. These properties make them desirable in items of jewellery.

3D molecular model of diamond showing strong network of bonds.

Diamond is extremely hard and has a high melting point. For this reason, it is very useful in cutting tools. The cutting edges of discs used to cut bricks and concrete are tipped with diamonds. Heavy-duty drill bits – such as those used in the oil exploration industry to drill through rocks – are made with diamonds so that they stay sharp for longer.

Diamond is insoluble in water. It does not conduct electricity. Every atom in a diamond is bonded to its neighbours by four strong covalent bonds, leaving no free electrons and no ions. This explains why diamond does not conduct electricity.

The bonding also explains the hardness of diamond and its high melting point. Significant quantities of energy would be needed to separate atoms so strongly bonded together.

Graphite

Graphite contains layers of carbon atoms.

3D molecular model of graphite.

Graphite is black, shiny and opaque. It is not transparent. It is also a very slippery material. It is used in pencil leads because layers easily slide onto the paper, leaving a black mark. It is a component of many lubricants, for example bicycle chain oil.

Graphite is insoluble in water. It has a high melting point and is a good conductor of electricity, which makes it a suitable material for the electrodes needed in electrolysis.

Each carbon atom is bonded into its layer with three strong covalent bonds. This leaves each atom with a spare electron, which together form a delocalised ‘sea’ of electrons loosely bonding the layers together. These delocalised electrons can all move along together – making graphite a good electrical conductor.

However, melting graphite is not easy. It takes considerable energy to break the strong covalent bonds and separate the carbon atoms.

Nanotubes

Nanotubes are a type of fullerene and are molecular-scale tubes of carbon arranged similarly to the layers in graphite.

3D molecular model of a carbon nanotube.

Carbon nanotubes have a very high melting point, as each carbon atom is joined to three other carbon atoms by strong covalent bonds. This also leaves each carbon atom with a spare electron, which forms a sea of delocalised electrons within the tube, meaning nanotubes can conduct electricity.