Silicon dioxide and giant covalent networks
Use network connectivity to explain why SiO₂ behaves more like diamond than carbon dioxide.
Exam scope and assessment notes
Extended only (Supplement) · Chemistry 0971, 2026–2028. Cambridge states that IGCSE 0971 has the same subject content as IGCSE 0620, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Explain diamond's three-dimensional covalent network.
- Distinguish a formula ratio from a separate molecule.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
How many carbon neighbours bond to one carbon in diamond?
What you will learn
- Describe the giant covalent structure of silicon(IV) oxide.
- Relate its similarities with diamond to their strong three-dimensional networks.
The formula gives a repeating ratio
Silicon(IV) oxide, SiO₂, is a giant covalent network. Each silicon is covalently bonded to four oxygen atoms, and each oxygen links two silicon atoms. This connectivity gives an overall ratio of one silicon to two oxygens: the oxygens are shared between neighbouring silicon centres. The formula does not represent isolated O=Si=O molecules in the solid.
A two-dimensional sketch is only a projection of the network. Do not infer that the material is a sheet like graphite simply because a drawing fits on paper. In the usual school model, the bonded network extends in three dimensions.
Compare the structure, not just the formula
Diamond and silicon dioxide both have strong covalent bonds throughout extended three-dimensional networks. Much energy is needed to overcome those bonds, so both have high melting points; their rigid networks also make them hard. Neither has mobile ions or a sea of delocalised electrons in this model, so both conduct electricity poorly.
Carbon dioxide has a different structure despite its similar-looking formula: it consists of separate simple molecules, with weaker attractions between molecules. A formula alone is insufficient to infer melting or electrical behaviour. Use the stated structure and explain the mechanism for each property.
Pause and explain
Which description fits solid silicon dioxide?
Worked example
Explain why SiO₂ has a much higher melting point than simple molecular CO₂.
Show the worked solution
- SiO₂ has strong covalent bonds throughout an extended network.
- Melting requires overcoming extensive strong bonding in that network.
- Separating CO₂ molecules requires overcoming much weaker intermolecular attractions.
Answer Network covalent bonds in SiO₂ require much more energy to overcome than attractions between CO₂ molecules.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Explain the Si:O ratio using four oxygens bonded around each silicon and two silicons bonded to each oxygen.
Give me a hint
Avoid counting every shared oxygen as belonging only to one silicon.
Compare my reasoning
- Count four Si–O connections from each silicon centre.
- Each oxygen participates in two connections by linking two silicon centres.
- Four connections per silicon divided by two per oxygen gives two oxygens per silicon.
The overall ratio is Si:O =1:2, although each silicon has four bonded oxygen neighbours.
Look for these in your work
- Shared oxygens are accounted for.
- Neighbour count is distinguished from whole-network formula ratio.
Give structure-based reasons why diamond and silicon dioxide both have high melting points and poor electrical conductivity.
Give me a hint
Explain energy requirements and charge mobility separately.
Compare my reasoning
- Both have strong covalent bonds throughout giant networks.
- Much energy is required to overcome extensive strong bonding for melting.
- Neither has mobile ions or freely moving delocalised electrons in this model.
Strong network bonds give high melting points; absence of mobile charge carriers gives poor conductivity.
Look for these in your work
- Both properties receive a mechanism.
- Conduction is not incorrectly explained by the material being hard.
A student predicts similar melting points for SiO₂ and CO₂ because both formulas contain two oxygens. Evaluate the reasoning.
Give me a hint
A formula gives composition; ask whether the particles are molecules or a network.
Compare my reasoning
- Similar formulas describe similar atom ratios, not necessarily similar structures.
- SiO₂ is a giant covalent network; CO₂ is simple molecular.
- Their different interactions during melting explain very different melting behaviour.
The prediction is unsupported: composition does not establish structure or melting behaviour.
Look for these in your work
- Composition and structure are distinguished.
- The energy comparison uses the interactions actually overcome.
Common mistakes
- Drawing isolated SiO₂ molecules to represent the giant structure.
- Assuming similar formulas guarantee similar physical properties.
What can you explain now?
How many silicon atoms does one bridging oxygen bond to in the network model?
Compare with the explanation
Two
Each oxygen joins two silicon centres; each silicon bonds to four oxygens, giving the overall Si:O ratio 1:2.
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Make it stick
Explain how SiO₂ can have four oxygen neighbours around each silicon while its formula still has two oxygens per silicon. Then compare it with CO₂ without relying on formulas alone.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.