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.

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.

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.