Explain why silicon dioxide has a high melting point and does not conduct electricity.

Edexcel GCSE Chemistry (1CH0) — 1.7 Types of substance · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Silicon dioxide is used to line the inside of furnaces because it can withstand very high temperatures. It is also used as an electrical insulator in some electronic components.

Model answer (4 marks)

Silicon dioxide has a giant covalent (macromolecular) structure, with each Si atom bonded to four O atoms and each O bonded to two Si atoms. The structure contains many strong covalent bonds throughout.

Because the bonds are covalent and the network is extensive, a large amount of energy is required to break them, giving SiO₂ a very high melting point.

There are no free or delocalised electrons, and no mobile ions are present, so the material cannot conduct electricity.

Examiner tips

Use the phrase "giant covalent structure" to show you know the structure type.
Mention the need for large energy to break covalent bonds to justify the high melting point.
State that there are no free electrons or mobile ions to explain the lack of conductivity.
Keep the answer concise and use the exact terminology from the mark scheme.

Common mistakes

Describing SiO₂ as ionic or metallic, which is incorrect.
Failing to mention the absence of free electrons or mobile ions.
Giving a long, unfocused explanation that repeats the same point.

Mark scheme (4 marks)

Silicon dioxide has a giant covalent (macromolecular) structure
There are many strong covalent bonds throughout the structure
A large amount of energy is needed to break these bonds, so the melting point is high
There are no free/delocalised electrons (and no ions free to move), so it cannot conduct electricity

Key terms in this question

melting point