Giant Covalent Structures

This lesson covers: 

  1. What giant covalent structures are
  2. Diamond - bonding, structure, properties
  3. Graphite - bonding, structure, properties

Giant covalent structures

Some elements can form extensive interconnecting networks of covalently bonded atoms known as giant covalent structures.

  • These structures involve huge lattices extending in three dimensions.
  • In carbon, the small atomic size and ability to form 4 covalent bonds per atom allow the formation of giant covalent structures.
  • The different structural forms of an element in the same state are called allotropes.


The 2 allotropes of carbon with giant lattice structures that you need to know about are diamond, and graphite.

Diamond allotrope

Bonding and structure:

Each carbon atom forms 4 very strong covalent bonds with others in a tetrahedral arrangement.

Diagram showing the tetrahedral arrangement of carbon atoms in a diamond allotrope.

Properties:

  1. Extremely hard - Extensive network of strong covalent bonds not easily broken
  2. Very high melting point - Huge amount of energy needed to break enough bonds to melt diamond
  3. Good thermal conductor - Strong interatomic bonds transmit heat through vibrations
  4. Electrical insulator - All outer electrons tied up in localised bonds so no free electrons to carry charge
  5. Insoluble - Covalent bonds too strong to be broken by solvation

Graphite allotrope

Bonding and structure:

  • Each carbon atom forms 3 strong covalent bonds in a planar hexagonal pattern, with each carbon contributing 1 delocalised electron.
  • Multiple stacked layers of hexagonal carbon arrays with weak intermolecular forces between layers.
Diagram showing the structure of graphite allotrope with strong covalent bonds, weak intermolecular forces, and delocalised electrons.

Properties:

  1. Soft and slippery - Weak intermolecular forces let sheets slide over each other
  2. Conducts electricity along layers - Delocalised electrons move through the 2D lattice carrying electrical charge
  3. Lower density than diamond - Weak intermolecular forces lead to increased separation between layers
  4. High sublimation temperature but lower melting point than diamond - Covalent bonds within each layer are very strong but the weaker intermolecular forces between layers means graphite melts at a lower temperature

Summary

The table below summarises the bonding, structure and properties of diamond and graphite.

DiamondGraphite
Bonding4 strong 3D covalent bonds per carbon atom in arrangement3 strong planar covalent bonds + 1 delocalised electron per carbon. Weaker interlayer forces
Structure3D network of tetrahedrally bonded carbon atomsStacked 2D hexagonal carbon sheets
PropertiesExtremely hard / Very high melting point / Good thermal conductor / Electrical insulator / InsolubleSofter and layers slide / Conducts electricity in layers / Lower density than diamond / High sublimation temperature