Giant Covalent StructuresAQA GCSE Chemistry: Revision notes
Section 1
What is a giant covalent structure?
A giant covalent structure is a substance in which very large numbers of atoms are joined together by strong covalent bonds throughout the whole structure, not just within small, separate molecules.
Because all the atoms are linked by strong covalent bonds in every direction, giant covalent structures have very high melting points — a huge amount of energy is needed to break the many strong bonds. Examples include diamond, graphite and silicon dioxide.
Section 2
What are the properties of diamond?
In diamond, each carbon atom forms four covalent bonds, creating a rigid, three-dimensional giant covalent structure.
This gives diamond its characteristic properties:
- Very hard — the rigid network of strong bonds resists being scratched or reshaped
- Very high melting point — many strong covalent bonds must be broken
- Does not conduct electricity — all electrons are held tightly in covalent bonds, with none free to move
Diamond's hardness comes directly from its four bonds per carbon atom — more bonds per atom than graphite, which only forms three.
Section 3
What are the properties of graphite?
In graphite, each carbon atom forms only three covalent bonds, arranged in layers of hexagonal rings. There are no covalent bonds between the layers.
This structure gives graphite its properties:
- Layers can slide over each other, since there are no strong bonds holding layers together — this is why graphite is soft and used as a lubricant
- Each carbon atom has one delocalised electron, similar to a metal, which is free to move and carry a charge
- This makes graphite conduct electricity, unlike diamond — it is used in electrodes
Don't say graphite conducts because it is metallic — it is still covalently bonded, but its delocalised electrons behave similarly to those in a metal.
Section 4
What are graphene and fullerenes?
Graphene is a single layer of graphite — just one atom thick. It is useful in electronics and composite materials because of its strength and ability to conduct electricity.
Fullerenes are molecules of carbon atoms with hollow shapes based mainly on hexagonal rings, though they may also contain rings of five or seven carbon atoms.
- Buckminsterfullerene (C60) has a spherical shape and was the first fullerene discovered
- Carbon nanotubes are cylindrical fullerenes with very high length-to-diameter ratios, used in nanotechnology, electronics and to strengthen materials
Must Know
- Giant covalent structures have very high melting points because all atoms are linked by strong covalent bonds throughout
- Diamond: each carbon forms 4 bonds — very hard, high melting point, does not conduct electricity
- Graphite: each carbon forms 3 bonds in layers with no bonds between layers — soft, slippery, conducts electricity via delocalised electrons
- Graphene is a single layer of graphite; useful in electronics and composites
- Fullerenes (including buckminsterfullerene C60 and carbon nanotubes) are hollow carbon structures based on hexagonal rings
That's the notes covered.
Carry on to the next subtopic.