AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Covalent bonding: Some covalently bonded substances have giant covalent structures, such as diamond and silicon dioxide.
Most covalently bonded substances are simple molecules with weak forces between them, but some form giant covalent structures in which every atom is joined to its neighbours by strong covalent…
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Most covalently bonded substances are simple molecules with weak forces between them, but some form giant covalent structures in which every atom is joined to its neighbours by strong covalent bonds throughout the whole solid. Diamond is a giant covalent structure of carbon atoms, each bonded to four others in a tetrahedral network, giving it extreme hardness and a very high melting point. Silicon dioxide (SiO₂) is also giant covalent: each silicon atom bonds to four oxygen atoms and each oxygen to two silicon atoms, forming a continuous three-dimensional network. Because melting requires breaking many strong covalent bonds, these substances have high melting points and do not conduct electricity, as there are no free electrons or ions.
What earns the marks
- Giant covalent structures contain many atoms joined by strong covalent bonds extending continuously through the substance, not discrete small molecules.
- In diamond, each carbon atom forms four covalent bonds to four other carbon atoms in a rigid tetrahedral network.
- In silicon dioxide, each silicon atom bonds to four oxygen atoms and each oxygen atom bonds to two silicon atoms, giving the formula SiO₂.
- Melting a giant covalent substance requires breaking many strong covalent bonds, so melting points are very high.
- Giant covalent substances such as diamond and silicon dioxide do not conduct electricity because they contain no free electrons or ions.
Common mistakes and how to put them right
Saying diamond is a molecule of carbon.
Put it right by describing it as a giant covalent lattice of many carbon atoms joined by covalent bonds.
Stating that giant covalent substances melt easily because covalent bonds are weak.
Put it right by explaining that individual covalent bonds are strong and many must be broken to melt the solid.
Describing silicon dioxide as simple molecules of SiO₂.
Put it right by explaining it is a giant covalent network in which the ratio of silicon to oxygen atoms is 1:2.
Exam tips
- When asked why diamond has a high melting point, link the answer to breaking many strong covalent bonds rather than to intermolecular forces.
- Use a labelled diagram of the diamond tetrahedral arrangement or the SiO₂ network to support your written explanation.
- Check whether the question asks about structure, bonding or properties, and make sure each part of your answer addresses the correct one.
Try the lesson
The start of the Covalent bonding: Some covalently bonded substances have giant covalent structures, such as diamond and silicon dioxide. lesson, made from checked teaching notes for this specification point.
Have a go first
Which of these substances has a giant covalent structure?
Listen
Most covalently bonded substances are simple molecules with weak forces between them, but some form giant covalent structures in which every atom is joined to its neighbours by strong covalent bonds throughout the whole solid. Diamond is a giant covalent structure of carbon atoms, each bonded to four others in a tetrahedral network, giving it extreme hardness and a very high melting point. Silicon dioxide (SiO₂) is also giant covalent: each silicon atom bonds to four oxygen atoms and each oxygen to two silicon atoms, forming a continuous three-dimensional network. Because melting requires breaking many strong covalent bonds, these substances have high melting points and do not conduct electricity, as there are no free electrons or ions.
Key words
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Giant covalent structures
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Melting points
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Electrical conductivity
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See every definition
- Giant covalent structures
- Structures in which every atom is joined to its neighbours by strong covalent bonds throughout the whole solid, such as diamond and silicon dioxide.
- Melting points
- Very high for giant covalent substances because melting requires breaking many strong covalent bonds.
- Electrical conductivity
- Giant covalent substances such as diamond and silicon dioxide do not conduct electricity because they contain no free electrons or ions.
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