Giant covalent structures of carbonEdexcel International A Level Chemistry: Revision notes
Section 1
Diamond
In diamond each carbon atom forms four strong covalent bonds to four other carbon atoms, arranged tetrahedrally. This gives a rigid giant covalent lattice.
- Very hard and with a very high melting point (over 3500 °C): strong bonds in all directions must be broken
- Does not conduct electricity: all four outer electrons are in localised bonds, so there are no mobile charge carriers
- Very high density (3.51 g cm⁻³) because the atoms are close-packed
Uses: cutting and drilling tools, abrasives, and jewellery.
Diamond does not conduct because its electrons are all held in bonds, not because it has no electrons.
Section 2
Graphite
In graphite each carbon atom forms three strong covalent bonds to three others, in flat hexagonal layers. The fourth outer electron is delocalised and free to move along the layer.
- Conducts electricity along the layers because of the delocalised electrons
- Soft and slippery: the layers are held by weak forces and slide over each other
- Very high melting point: the strong covalent bonds within the layers must be broken
- Lower density (2.27 g cm⁻³) than diamond because the layers are far apart (0.335 nm) compared with the bond length in a layer (0.142 nm)
Uses: pencils, dry lubricants and electrodes.
Do not say covalent bonds break when graphite is a lubricant. Only the weak forces between layers are overcome, so the layers slide.
Section 3
Graphene
Graphene is a single layer of graphite, one atom thick, with each carbon atom bonded to three others in a hexagonal network and one delocalised electron per atom.
- Conducts electricity very well across the sheet
- Very strong for its mass because of the strong covalent bonds, yet flexible
- Almost transparent because it is only one atom thick
Uses: transparent flexible touch screens, lightweight strong composites, and electronic devices.
Link each use to a property and each property to the structure: use, property, bonding. For example, touch screen, transparent conductor, one atom thick with delocalised electrons.
Section 4
Comparing the three forms
- Diamond: 4 bonds per carbon, tetrahedral lattice, does not conduct, very hard
- Graphite: 3 bonds per carbon, layers, conducts, soft
- Graphene: 3 bonds per carbon, single layer, conducts, strong sheet
All three have strong covalent bonds. The differences come from the number of bonds per atom, which decides whether an electron is delocalised, and from the arrangement, which decides whether layers can slide.
Must know
- Diamond: four bonds per carbon, hard, non-conductor, cutting tools
- Graphite: three bonds, layers with weak forces, conducts, lubricant and electrodes
- Graphene: single layer, conducts, transparent, strong, touch screens and composites
- Link every property to bonding and structure
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Giant covalent structures of carbon
- A drilling company fits diamond-tipped bits to its rock drills. The same company also supplies graphite for use as a dry lubricant and as electrodes.Explain why graphite is soft and can be used as a lubricant.2 marks
- Graphene is a single layer of graphite, one atom thick, in which carbon atoms are arranged in a hexagonal network. It is being developed for transparent, flexible touch screens and for strong, lightweight composite materials.Explain why graphene conducts electricity.2 marks
- Diamond and graphite are both forms of pure carbon and both have very high melting points. Diamond has a density of 3.51 g cm⁻³ and does not conduct electricity. Graphite has a density of 2.27 g cm⁻³ and does conduct electricity. In graphite the distance between neighbouring layers is 0.335 nm, which is more than twice the carbon–carbon bond length within a layer, 0.142 nm.Explain, in terms of structure and bonding, why diamond has a very high melting point and does not conduct electricity.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).