Revision notes for AQA GCSE Chemistry Giant covalent structures. Open the guide for explanations and worked examples. Written against the AQA GCSE Chemistry (8462) specification, so the content matches what's examinable rather than general Chemistry background.
Giant covalent structures
What you'll learn
- What a giant covalent structure is, and how it differs from a small molecule.
- Why giant covalent substances are solids with very high melting points.
- How to recognise diamond, graphite and silicon dioxide from bonding diagrams.
- How to write clear structure-and-property explanations in GCSE answers.
Before we start: bonds, structures and properties
An atom is the smallest part of an element that can take part in chemical reactions. An element is a substance made from only one type of atom, such as carbon.
A physical property is something you can observe or measure without making a new substance, such as melting point, hardness or electrical conductivity.
Definition
Covalent bond
A covalent bond is a strong bond formed when two atoms share a pair of electrons. In GCSE explanations, the key idea is: covalent bonds are strong.
A structure means the arrangement of particles in a substance. In this topic, the particles are atoms joined by covalent bonds.
Simple molecules versus giant covalent structures
Some covalent substances are made of small molecules. A molecule is a group of atoms joined together by covalent bonds. For example, oxygen gas contains O₂ molecules and water contains H₂O molecules.
In a simple molecular substance, the covalent bonds inside each molecule are strong, but the forces between separate molecules are weak. These weak attractions are called intermolecular forces.
Giant covalent structures are different: there are no small, separate molecules. Instead, atoms are joined in a huge repeating network.
Definition
Giant covalent structure
A giant covalent structure is a substance made from a very large number of atoms joined together by strong covalent bonds in a repeating network.
The word giant does not mean the atoms are large. It means the bonded network continues through the whole solid.
Common Mistake
Thinking all covalent substances have low melting points
Some covalent substances have low melting points because they are simple molecular. Giant covalent substances have very high melting points because strong covalent bonds extend throughout the structure.
Example
Deciding whether a covalent diagram shows a giant structure
A diagram shows many atoms joined in a repeating pattern, with bonds continuing off the edge of the diagram. Is it likely to be a giant covalent structure?
- Check whether the atoms are in separate small groups or one continuous network. Here, the atoms are joined together in one repeating pattern.
- Notice that bonds continue off the edge of the diagram. This suggests the structure keeps going beyond the part shown.
- Conclude that it is a giant covalent structure, not a simple molecular substance.
Why giant covalent substances have very high melting points
The melting point is the temperature at which a solid changes into a liquid.
Giant covalent substances are usually solids at room temperature and have very high melting points. This is because to melt them, many strong covalent bonds must be overcome.
In a simple molecular substance, melting only overcomes weak intermolecular forces between molecules. In a giant covalent substance, melting involves breaking strong covalent bonds in the network.
Key Idea
The high melting point explanation
Giant covalent structures have very high melting points because many strong covalent bonds must be broken, which requires a large amount of energy.
Example
Explaining a high melting point
Silicon dioxide has a very high melting point. Explain why.
- Identify the structure: silicon dioxide has a giant covalent structure, so it is not made from separate small molecules.
- Identify the bonds that must be overcome: the structure contains many strong covalent bonds between atoms.
- Link to energy: a large amount of energy is needed to break these bonds, so the melting point is very high.
Recognising giant covalent structures from diagrams
In exams, you may be shown a two-dimensional drawing of a three-dimensional structure. Two-dimensional means flat, with height and width. Three-dimensional means it also has depth.
Look for these clues:
- A repeating pattern of atoms.
- Atoms joined by covalent bonds in a continuous network.
- Bonds going off the edge of the diagram, showing the structure continues.
- No separate small molecules.
- Many atoms bonded to several neighbouring atoms.
Tip
How to spot a giant covalent diagram
Do not just count how many atoms are drawn. Ask: are the atoms separate molecules, or one continuous bonded network?
Key examples you need to know
Diamond, graphite and silicon dioxide are examples of giant covalent structures. Diamond and graphite are both forms of carbon.
An allotrope is a different structural form of the same element. Diamond and graphite are allotropes of carbon because they both contain only carbon atoms, but their atoms are arranged differently.
Diamond
Diamond is a giant covalent structure made only from carbon atoms.
Each carbon atom in diamond forms covalent bonds with four other carbon atoms. This makes a strong three-dimensional network. Because the covalent bonds go in all directions, diamond is very hard and has a very high melting point.
Graphite
Graphite is also made only from carbon atoms, but its structure is different from diamond.
In graphite, each carbon atom is covalently bonded to three other carbon atoms in flat hexagonal layers. A hexagon is a six-sided shape. The covalent bonds within each layer are strong.
There are weak forces between the layers, so the layers can slide over each other. This explains why graphite is soft and slippery. Graphite also has delocalised electrons, which are electrons that are free to move through part of the structure. This helps graphite conduct electricity.
The diagram below compares diamond and graphite, showing why both are giant covalent structures even though their arrangements are different.
Common Mistake
Calling graphite simple molecular
Graphite is not simple molecular. Even though its layers can slide, each layer is a giant covalent network of carbon atoms.
Example
Comparing diamond and graphite
Diamond and graphite are both made from carbon, but they have different properties. Explain why.
- Compare the bonding: in diamond, each carbon atom bonds to four other carbon atoms; in graphite, each carbon atom bonds to three other carbon atoms in layers.
- Compare the structure: diamond has a strong three-dimensional network; graphite has strong covalent bonds within layers but weak forces between layers.
- Link to properties: diamond is very hard because strong covalent bonds act in all directions, while graphite is soft because the layers can slide over each other.
Silicon dioxide
Silicon dioxide is also called silica. It is a compound, which means it contains atoms of two or more different elements chemically bonded together. Silicon dioxide contains silicon and oxygen atoms.
Silicon dioxide has a giant covalent structure. In the network, silicon and oxygen atoms are joined by strong covalent bonds. The important GCSE point is that silicon dioxide is not made from separate SiO₂ molecules in the solid; it is a giant network.
The diagram below shows silicon dioxide as a repeating covalent network.
Because many strong silicon-oxygen covalent bonds must be broken to melt silicon dioxide, it has a very high melting point. This is why silica is useful in materials such as glass.
Writing good GCSE explanations
When a question asks you to explain a property, do not stop at naming the structure. Link the structure to the bonds, then link the bonds to the property.
A strong answer usually follows this chain:
- Name the structure: giant covalent structure.
- Describe the bonding: many strong covalent bonds between atoms.
- Explain the energy change: a lot of energy is needed to break the bonds.
- State the property: very high melting point.
Exam technique
In the exam
- If asked about melting point, always mention many strong covalent bonds and a large amount of energy.
- If shown a diagram, decide whether it shows separate molecules or a continuous repeating network.
- Avoid saying “strong intermolecular forces” for diamond, graphite or silicon dioxide — the key bonds are covalent bonds.
Self review
Check yourself
- Why do giant covalent substances have very high melting points?
- How can you tell from a diagram that a substance is giant covalent rather than simple molecular?
- What is the main structural difference between diamond and graphite?
How bonding and structure are related to the properties of substances
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