Silicon dioxide is a giant covalent structure. Explain why silicon dioxide has a high melting point and does not conduct electricity in any state.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Silicon dioxide is used to line furnaces and as an electrical insulator in high-temperature applications. Each silicon atom is covalently bonded to four oxygen atoms, and each oxygen atom is covalently bonded to two silicon atoms throughout the structure.
Model answer (4 marks)
Silicon dioxide is a giant covalent network; every Si atom is bonded to four O atoms and each O to two Si atoms, giving a continuous lattice of strong covalent bonds.
Because the entire structure is held together by these covalent bonds, a large amount of energy is required to break them and convert the solid into a liquid. This high bond energy results in a very high melting point.
In a covalent network there are no free or delocalised electrons; the electrons are shared only within the covalent bonds. Consequently there are no mobile charged particles to carry a current, so silicon dioxide does not conduct electricity in any state.
Because the entire structure is held together by these covalent bonds, a large amount of energy is required to break them and convert the solid into a liquid. This high bond energy results in a very high melting point.
In a covalent network there are no free or delocalised electrons; the electrons are shared only within the covalent bonds. Consequently there are no mobile charged particles to carry a current, so silicon dioxide does not conduct electricity in any state.
Examiner tips
- Mention the giant covalent network first; link it to bond strength and melting point. State that electrons are not delocalised, explaining the lack of conductivity. Use the exact terms "covalent bonds", "bond energy", "free/delocalised electrons" as per the mark scheme.
Common mistakes
- Confusing silicon dioxide with ionic compounds and claiming it has a lattice of ions. Saying it conducts electricity because it is a solid. Failing to mention the absence of free electrons or the need for bond breaking to melt.
Mark scheme (4 marks)
- Silicon dioxide contains many strong covalent bonds throughout the structure
- A large amount of energy is needed to break these covalent bonds, so the melting point is high
- There are no free/delocalised electrons in silicon dioxide
- Therefore there are no charged particles free to move to carry electrical charge/current
Key terms in this question
Related
- All Pearson Edexcel International GCSE Chemistry (4CH1) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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