Silicon dioxide is a giant covalent structure used to make glass and heat-resistant laboratory equipment. Explain why silicon dioxide has a 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 found naturally as quartz and is used in applications that require resistance to very high temperatures. Unlike metals, it does not conduct electricity in any state.
Model answer (4 marks)
Silicon dioxide is a giant covalent network crystal.
1. Each Si atom is tetrahedrally bonded to four O atoms and each O to two Si atoms, forming a continuous three‑dimensional lattice.
2. The bonds are strong covalent bonds; a large amount of energy is required to break them, giving SiO₂ a very high melting point.
3. In this network there are no free or delocalised electrons; all valence electrons are involved in the covalent bonds.
4. Because charge carriers are absent, the structure cannot conduct electricity.
1. Each Si atom is tetrahedrally bonded to four O atoms and each O to two Si atoms, forming a continuous three‑dimensional lattice.
2. The bonds are strong covalent bonds; a large amount of energy is required to break them, giving SiO₂ a very high melting point.
3. In this network there are no free or delocalised electrons; all valence electrons are involved in the covalent bonds.
4. Because charge carriers are absent, the structure cannot conduct electricity.
Examiner tips
- Use the word "giant covalent network" to show understanding of structure. Explain that high bond energy → high melting point. State absence of free electrons → no conductivity. Keep answer concise to fit 4 marks.
Common mistakes
- Confusing SiO₂ with ionic compounds and claiming it conducts electricity. Forgetting to mention the covalent nature of the bonds. Using vague phrases like "strong bonds" without linking to melting point.
Mark scheme (4 marks)
- Silicon dioxide contains many atoms held together by strong covalent bonds
- A large amount of energy is needed to break these strong covalent bonds, so the melting point is high
- There are no free/delocalised electrons in silicon dioxide
- Charge cannot be carried through the structure, so it does not conduct electricity
Key terms in this question
giant covalent structure · melting point
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