Silicon dioxide (SiO₂) is used to make glass and optical fibres. It has a very high melting point and does not conduct electricity. Explain the properties of silicon dioxide in terms of its giant covalent structure.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Silicon dioxide is a giant covalent structure used widely in industry. Unlike metals or ionic compounds, it behaves very differently when heated or when a voltage is applied.
Model answer (5 marks)
Silicon dioxide is a giant covalent network crystal in which each silicon atom is covalently bonded to four oxygen atoms and each oxygen to two silicon atoms, forming a continuous three‑dimensional lattice.
The bonds in this lattice are covalent and very strong, so a large amount of energy is required to break them. This high bond strength accounts for the very high melting point of SiO₂.
Because all electrons are involved in covalent bonds, there are no free electrons or ions to carry charge, so SiO₂ does not conduct electricity.
The bonds in this lattice are covalent and very strong, so a large amount of energy is required to break them. This high bond strength accounts for the very high melting point of SiO₂.
Because all electrons are involved in covalent bonds, there are no free electrons or ions to carry charge, so SiO₂ does not conduct electricity.
Examiner tips
- Use the phrase "giant covalent network" to show understanding of structure.
- Mention bond strength and energy required to break bonds to justify high melting point.
- Explain lack of delocalised electrons to justify non‑conductivity.
Common mistakes
- Confusing SiO₂ with an ionic compound and claiming it conducts electricity.
- Omitting the link between bond strength and high melting point.
- Using vague terms like "strong bonds" without explaining the energy requirement.
Mark scheme (5 marks)
- Silicon dioxide contains many atoms held together by covalent bonds
- The covalent bonds are strong
- A large amount of energy is needed to break these strong covalent bonds
- This explains the high melting point of silicon dioxide
- Silicon dioxide does not conduct electricity because it has no delocalised electrons (or free ions) to carry charge
Key terms in this question
giant covalent structure · melting point
Related
- All OCR A-Level Chemistry A (H432) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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