Explain why silicon dioxide (SiO₂) has a very high melting point and does not conduct electricity.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Silicon dioxide is used to make glass and is found in sand. It is a solid at room temperature with a melting point above 1600 °C.
Model answer (5 marks)
Silicon dioxide has a giant covalent structure.
Each silicon atom is bonded to four oxygen atoms by strong covalent bonds, forming a continuous 3‑D network.
The network is held together by many strong Si–O bonds, so a large amount of energy is required to break the structure and melt the solid.
Because the electrons are tightly bound in covalent bonds, there are no free (delocalised) electrons.
There are also no mobile ions in the solid, so SiO₂ does not conduct electricity.
Each silicon atom is bonded to four oxygen atoms by strong covalent bonds, forming a continuous 3‑D network.
The network is held together by many strong Si–O bonds, so a large amount of energy is required to break the structure and melt the solid.
Because the electrons are tightly bound in covalent bonds, there are no free (delocalised) electrons.
There are also no mobile ions in the solid, so SiO₂ does not conduct electricity.
Examiner tips
- Use the term "giant covalent structure" early. Explain that the high melting point comes from the need to break many strong covalent bonds. State both reasons for lack of conductivity: no free electrons and no mobile ions.
Common mistakes
- Confusing SiO₂ with ionic compounds and claiming it conducts. Forgetting to mention the lack of mobile ions. Using vague phrases like "strong bonds" without linking to the high melting point.
Mark scheme (5 marks)
- Silicon dioxide has a giant covalent structure (giant lattice / macromolecular structure)
- Each silicon atom is bonded to oxygen atoms by strong covalent bonds throughout the structure
- A large amount of energy is needed to break these strong covalent bonds
- Silicon dioxide does not conduct electricity because it has no free (delocalised) electrons
- Silicon dioxide does not conduct electricity because it has no mobile ions (in the solid or liquid state)
Related
- All AQA A-Level Chemistry (7405) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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