Giant covalent structures
4 marks · NormalThe table shows some physical properties of diamond and graphite.
| Substance | Melting point/°C | Hardness | Conducts electricity |
|---|---|---|---|
| Diamond | 3550 | Very hard | No |
| Graphite | 3600 | Soft | Yes |
Explain, in terms of their structures and bonding, why diamond is much harder than graphite.
Explain, in terms of bonding, why graphite conducts electricity but diamond does not.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) Diamond is harder because each carbon atom forms four strong covalent bonds, creating a rigid three-dimensional lattice. In graphite, strong bonds only hold atoms in layers, but weak forces act between layers, allowing them to slide and making graphite soft.
(b) Graphite conducts electricity because it has delocalised electrons that can move through the layers. Diamond does not conduct electricity as all its outer electrons are used in covalent bonds, so there are no free electrons.
Common mistakes: not mentioning type of bonding; missing explanation for delocalised electrons
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Marking points
M1 (a) · 1 mark · Refers to the 3D lattice/strong covalent bonds in diamond making it hard; describes layered structure and weak forces in graphite making it soft.
A1 (a) · 1 mark · Correct comparison: diamond's structure gives hardness vs. graphite's layers slide.
M2 (b) · 1 mark · Identifies delocalised electrons in graphite.
A2 (b) · 1 mark · States diamond has no free electrons, so does not conduct; graphite conducts as electrons are free to move.
Worked solution
a) Diamond is much harder because its structure is a giant 3D lattice where each carbon atom forms four strong covalent bonds throughout the structure. This makes it rigid and difficult to break. Graphite has strong covalent bonds only within layers, but the forces between layers are weak—so layers slide, making graphite soft.
b) In graphite, each carbon atom has one delocalised electron per atom that is free to move and carry charge, so graphite conducts electricity. In diamond, all four outer electrons are involved in covalent bonds; there are no free electrons, so it does not conduct electricity.