2.6 Giant covalent structures
- Syllabus
- 0620–2026–2027
- Topic
- 2.6
- Level
- —
Learning objectives
2.6.1Giant covalent structures of graphite• Describe giant covalent structures of graphite and diamond2.6.2Relate the structures and bonding of• Relate the structures and bonding of graphite and diamond to their uses, (a) graphite as a lubricant and as an electrode (b) diamond in cutting tools2.6.3Giant covalent structure of• Describe giant covalent structure of silicon(IV) oxide, SiO22.6.4Similarity in properties between• Describe similarity in properties between diamond and silicon(IV) oxide, related to their structures
Compare the structures of diamond and graphite
Diamond and graphite are different forms of carbon. Both are giant covalent structures, but their carbon atoms are connected differently.
| Feature | Diamond | Graphite |
|---|---|---|
| bonds from each carbon atom | four strong covalent bonds | three strong covalent bonds |
| arrangement | rigid three-dimensional network | flat hexagonal layers |
| remaining outer electron | none available for movement | one per carbon is delocalised and can move along layers |
| between structural units | continuous bonds in all directions | weak attractions between layers |
Neither structure consists of separate carbon molecules. The covalent network extends through a giant number of atoms; a drawn model shows only a small repeating part.
Graphite's covalent bonds within each layer are strong. Only the attractions between different layers are weak.
Link diamond and graphite structures to uses
A useful property must be linked to the exact structural feature that causes it.
| Material and use | Structural feature | Resulting property |
|---|---|---|
| graphite as a lubricant | layers have only weak attractions between them | layers slide over one another easily |
| graphite as an electrode | each carbon contributes a delocalised electron that can move along layers | conducts electricity |
| diamond in cutting tools | every carbon has four strong covalent bonds in a rigid 3D network | extremely hard and resists deformation |
Strong covalent bonding throughout both structures also gives high thermal stability, so the giant framework is not easily separated by heating.
Graphite conducts because of mobile delocalised electrons, not mobile ions. Diamond has no such available electrons and does not conduct electricity.
Describe the giant structure of silicon(IV) oxide
Silicon(IV) oxide, SiO₂, has a giant covalent structure: a continuous three-dimensional network of silicon and oxygen atoms joined by strong covalent bonds.
| Atom | Covalent connections in the network |
|---|---|
| each silicon atom | bonded to four oxygen atoms |
| each oxygen atom | bonded to two silicon atoms |
| overall ratio | one Si for every two O, giving SiO₂ |
The structure contains no separate SiO₂ molecules. The formula gives the simplest atom ratio in the giant network, and the covalent bonds continue throughout the solid.
All outer electrons are held in covalent bonds, so there are no mobile ions or delocalised electrons in the structure.
The formula SiO₂ does not mean one silicon atom is bonded to only two oxygen atoms; shared oxygen atoms produce the 1:2 ratio across the network.
Relate diamond and silicon(IV) oxide
Diamond and silicon(IV) oxide have similar properties because both are rigid three-dimensional giant covalent networks.
| Shared property | Structural explanation |
|---|---|
| hard | many strong covalent bonds hold atoms in fixed positions through the network |
| high melting point | a large amount of energy is needed to break strong covalent bonds throughout the giant structure |
| poor electrical conductivity | there are no mobile ions or delocalised electrons to carry charge |
The networks contain different atoms—only carbon in diamond, silicon and oxygen in SiO₂—but both connect atoms by strong covalent bonds in a giant framework.
Their high melting points are not caused by intermolecular forces: neither substance consists of discrete molecules.