2.8 Giant covalent structures: diamond, graphite and silicon dioxide
6 exam questions worth 12 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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How many covalent bonds does each carbon atom form in graphite?
Choose one answer.
Correct Not quite: the answer is 3
Mark scheme
B
Model answer
B: three; the fourth outer electron is delocalised.
Explain why diamond is hard and is used for cutting tools.
Mark scheme
- each carbon atom forms four (strong) covalent bonds (1)
- giant / rigid 3D structure (1)
- lots of energy needed to break the bonds / no layers to slide (1)
Model answer
In diamond, every carbon atom is joined to four others by strong covalent bonds in a rigid giant structure. A lot of energy is needed to break these bonds, so diamond is very hard and can cut other materials.
Explain why graphite conducts electricity.
Mark scheme
- each carbon atom has one delocalised electron (1)
- which can move through the structure / along the layers (1)
Model answer
Each carbon atom is bonded to only three others, so one electron per atom is delocalised. These electrons can move through the layers, carrying charge.
Explain why graphite is soft and can be used as a lubricant.
Mark scheme
- layers are held by weak forces / no covalent bonds between layers (1)
- so the layers can slide over each other (1)
Model answer
There are only weak forces between the layers, so the layers slide over each other easily.
Explain why silicon dioxide has a very high melting point.
Mark scheme
- giant covalent structure (1)
- many strong covalent bonds must be broken, needing a lot of energy (1)
Model answer
Silicon dioxide is a giant covalent structure, so many strong covalent bonds must be broken to melt it, which needs a lot of energy.
Substance Y has a melting point of 1610 °C. It does not conduct electricity when solid or when molten.
What type of structure does Y have? Explain your answer.
Mark scheme
- giant covalent (1)
- very high melting point and does not conduct even when molten, so no ions or delocalised electrons (1)
Model answer
Y is giant covalent: its very high melting point means many strong bonds, and it does not conduct even when molten, so it is not ionic or metallic.
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