Giant Covalent Structures (Diamond, Graphite, Silicon Dioxide)

This lesson covers giant covalent structures as required by the AQA GCSE Combined Science Trilogy specification (8464), section 4.2.2. You need to know the structures and properties of diamond, graphite and silicon dioxide, and explain why their properties differ from simple molecular substances.

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What Is a Giant Covalent Structure?

A giant covalent structure (also called a macromolecular structure) is a huge network of atoms all joined together by strong covalent bonds. Unlike simple molecular substances, there are no small individual molecules — the entire structure is one continuous network of covalently bonded atoms.

Because all the bonds are strong covalent bonds that extend throughout the whole structure, giant covalent substances have very high melting and boiling points.

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Diamond

Structure

• Diamond is a form of carbon (C).
• Each carbon atom is covalently bonded to 4 other carbon atoms in a tetrahedral arrangement.
• The structure forms a rigid 3D lattice that extends in all directions.
• All outer-shell electrons are involved in bonding — there are no free electrons.

graph TD
    A["Diamond Structure"] --> B["Each C bonded to<br/>4 other C atoms"]
    A --> C["Tetrahedral<br/>arrangement"]
    A --> D["All electrons<br/>in covalent bonds"]
    B --> E["Very high<br/>melting point"]
    C --> F["Very hard<br/>substance"]
    D --> G["Does NOT<br/>conduct electricity"]

    style A fill:#2c3e50,color:#fff
    style B fill:#3498db,color:#fff
    style C fill:#3498db,color:#fff
    style D fill:#3498db,color:#fff
    style E fill:#27ae60,color:#fff
    style F fill:#27ae60,color:#fff
    style G fill:#e74c3c,color:#fff

Properties of Diamond

Property	Explanation
Very high melting point (3550°C)	Many strong covalent bonds must be broken, requiring a very large amount of energy
Very hard	The rigid 3D network of covalent bonds makes diamond extremely resistant to deformation
Does not conduct electricity	All four outer electrons on each carbon atom are used in covalent bonds — there are no free (delocalised) electrons
Insoluble in water	The strong covalent bonds throughout the structure cannot be broken by water molecules

Exam Tip: Diamond is the hardest natural substance and is used in cutting tools and drill bits because of its rigid 3D lattice structure.

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Graphite

Structure

• Graphite is another form of carbon — diamond and graphite are allotropes of carbon (different structural forms of the same element in the same physical state).
• Each carbon atom is covalently bonded to 3 other carbon atoms, forming flat hexagonal layers.
• This leaves one delocalised electron per carbon atom that is free to move between the layers.
• The layers are held together by weak intermolecular forces (van der Waals forces).

Properties of Graphite

Property	Explanation
Very high melting point (3652°C)	Many strong covalent bonds within the layers must be broken
Soft and slippery	The weak intermolecular forces between layers allow the layers to slide over each other easily
Conducts electricity	Each carbon bonds to only 3 others, leaving one delocalised electron per atom that is free to move and carry charge
Insoluble in water	Strong covalent bonds within layers cannot be broken by water

Exam Tip (AQA 8464): The key difference between diamond and graphite in terms of electrical conductivity is: in graphite, each carbon forms only 3 bonds (leaving 1 delocalised electron), whereas in diamond, each carbon forms 4 bonds (no free electrons).

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Diamond vs Graphite Comparison

Feature	Diamond	Graphite
Bonds per carbon atom	4	3
Structure	3D tetrahedral lattice	Flat hexagonal layers
Hardness	Very hard	Soft and slippery
Electrical conductivity	Does not conduct	Conducts (delocalised electrons)
Melting point	Very high	Very high
Delocalised electrons?	No	Yes (1 per carbon atom)
Forces between layers	N/A (no layers)	Weak intermolecular forces

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Silicon Dioxide (SiO₂)

Structure

• Silicon dioxide has a giant covalent structure similar to diamond.
• Each silicon atom is covalently bonded to 4 oxygen atoms.
• Each oxygen atom is bonded to 2 silicon atoms.
• The structure extends in a 3D network.

Properties of Silicon Dioxide

Property	Explanation
Very high melting point (1713°C)	Many strong covalent bonds must be broken
Very hard	Rigid 3D lattice, similar to diamond
Does not conduct electricity	No free electrons or ions — all electrons are involved in covalent bonds
Insoluble in water	Strong covalent bonds cannot be overcome by water

Exam Tip: Silicon dioxide (sand, quartz) has similar properties to diamond — very hard, very high melting point, does not conduct electricity. Remember it as "like diamond but with Si and O atoms."

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Giant Covalent vs Simple Molecular: Key Comparison

graph LR
    A["Covalent Substances"] --> B["Simple Molecular<br/>e.g. H₂O, CO₂, CH₄"]
    A --> C["Giant Covalent<br/>e.g. Diamond, Graphite, SiO₂"]
    B --> D["Low melting point<br/>Weak intermolecular forces"]
    B --> E["Do not conduct<br/>electricity"]
    C --> F["Very high melting point<br/>Many strong covalent bonds"]
    C --> G["Do not conduct<br/>(except graphite)"]

    style A fill:#2c3e50,color:#fff
    style B fill:#3498db,color:#fff
    style C fill:#e67e22,color:#fff
    style D fill:#3498db,color:#fff
    style E fill:#3498db,color:#fff
    style F fill:#e67e22,color:#fff
    style G fill:#e67e22,color:#fff

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Allotropes of Carbon

Allotropes are different structural forms of the same element in the same physical state. Carbon has several allotropes:

• Diamond — 3D tetrahedral lattice, each C bonded to 4 others
• Graphite — hexagonal layers, each C bonded to 3 others
• Graphene — a single layer of graphite (covered in the next lesson)
• Fullerenes — hollow carbon structures (covered in the next lesson)

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Common Mistakes