Giant covalent structures of carbon

Allotropes are different forms of an element in the same physical state. Diamond and graphite are allotropes of carbon. They both consist of giant covalent structures in which very many carbon atoms are joined together by covalent bonds. However, their detailed structures and bonding differ, so their physical properties are different.

Diamond

Structure and bonding

Diamond is a giant covalent substance in which:

  • each carbon atom is joined to four other carbon atoms by covalent bonds
  • the carbon atoms form a regular tetrahedral network structure
  • there are no free electrons
The structure of diamond.Carbon atoms in diamond form a 'tetrahedral' arrangement

Properties and uses

The three-dimensional arrangement of carbon atoms, held together by strong covalent bonds, makes diamond very hard. This makes it useful for cutting tools, such as diamond-tipped glass cutters and oil rig drills.

Diamond has a very high melting point because a large amount of energy is needed to overcome the many strong covalent bonds. There are no electrons or other charged particles that are free to move so diamond does not conduct electricity.

Graphite

Structure and bonding

Graphite is a giant covalent substance in which:

  • each carbon atom is joined to three other carbon atoms by covalent bonds
  • the carbon atoms form a hexagonal layered network structure
  • the layers have weak forces between them and can slide over each other
  • each carbon atom has one non-bonding outer electron
  • these non-bonding electrons are delocalised, and are free to move
The structure of graphite ball and stick diagram.Dotted lines model the weak forces between the layers in graphite

Properties and uses

Delocalised electrons are free to move through the structure of graphite, so graphite can conduct electricity. This makes it useful for electrodes in batteries and for electrolysis.

The layers in graphite can slide over each other because the forces between them are weak. This makes graphite slippery, so it is useful as a lubricant.

Graphite is used to make the core or 'lead' in pencils because it is soft. The layers are easily rubbed off to leave a mark on paper.