This section explores the unique giant covalent structures of two important allotropes of carbon: diamond and graphite. We will describe their structures and discuss the properties that arise from these structures.
Diamond
Structure
Diamond has a giant covalent structure where each carbon atom is covalently bonded to four other carbon atoms in a tetrahedral arrangement. This forms a strong, three-dimensional network.
Suggested diagram: A 3D representation of the diamond lattice showing tetrahedral bonding.
Description
In a diamond crystal, each carbon atom is strongly bonded to its four nearest neighbours. This creates a rigid, three-dimensional tetrahedral network. There are no weak forces between the diamond molecules.
Properties Arising from the Structure
Very high melting point: A large amount of energy is required to break the strong covalent bonds in the network.
Very high hardness: The strong covalent bonds throughout the structure make diamond extremely resistant to scratching.
Electrical insulator: All valence electrons are involved in covalent bonding and are not free to move, so it does not conduct electricity.
Transparent: The electrons are not available to absorb light, allowing light to pass through.
High refractive index: The large difference in dielectric constant between air and diamond causes light to bend significantly when passing through it.
Graphite
Structure
Graphite also consists of carbon atoms, but its structure is different from diamond. Carbon atoms are arranged in layers of hexagonal rings. Within each layer, each carbon atom is covalently bonded to three other carbon atoms. These layers are held together by weak Van der Waals forces.
Suggested diagram: A layered structure of graphite showing hexagonal rings and weak Van der Waals forces between layers.
Description
Graphite consists of layers of carbon atoms arranged in hexagonal rings. Each carbon atom in a ring is covalently bonded to three other carbon atoms within the same ring. These layers are weakly held together by forces between the layers.
Properties Arising from the Structure
Low melting point: The weak Van der Waals forces between the layers are easily overcome by heat.
Softness: The layers can easily slide past each other due to the weak forces between them.
Electrical conductivity: The delocalised electrons within each layer can move freely, allowing graphite to conduct electricity along the layers.
Transparency: Light can pass through the layers of graphite.
Lubricant: The layers can easily slide past each other, making graphite a good lubricant.
Comparison Table
Property
Diamond
Graphite
Structure
Giant tetrahedral covalent network
Layers of hexagonal rings held by weak Van der Waals forces