Lesson overview
Giant structures: Why is diamond hard but graphite slippery?
Diamond and graphite are both pure carbon, yet one cuts stone and the other rubs off onto paper. Predict how they will behave, put a diamond, graphite, quartz and copper through three tests, scratching glass, lighting a bulb and taking a hammer blow, and look at each one through the magnifier for how its atoms are joined.
Suggested sequence
- Lesson 1 · Diamond and graphite. The giant covalent structures of diamond and graphite and their uses; Supplement: silicon(IV) oxide and why it is like diamond. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- Lesson 2 · A sea of electrons. Supplement: metallic bonding, and why metals conduct and can be hammered and drawn into wires. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- End of module · final check. 2 new questions, 20–30 min, without hints or notes; pupils check their answers once they have answered them all.
- One to two weeks later · return questions. 2 questions, 15 min, lesson closed.
Learning goals
Part 1: Diamond and graphite
- Describe the giant covalent structures of graphite and diamond
- Relate the structures and bonding of graphite and diamond to their uses, limited to: (a) graphite as a lubricant and as an electrode (b) diamond in cutting tools
- Describe the giant covalent structure of silicon(IV) oxide, SiO2
- Describe the similarity in properties between diamond and silicon(IV) oxide, related to their structures
Part 2: A sea of electrons
- Describe metallic bonding as the electrostatic attraction between the positive ions in a giant metallic lattice and a ‘sea’ of delocalised electrons
- Explain in terms of structure and bonding the properties of metals: (a) good electrical conductivity (b) malleability and ductility
Misconceptions to listen for
- “Count the bonds of an atom at the edge of the diagram.” — An edge atom has dashed stubs where the network goes on. Count the bonds of an atom in the middle.
- “Two, one to each of two other carbon atoms.” — Some atoms at the edge of the diagram show only two solid bonds, but they also have dashed stubs. An atom in the middle, with all its neighbours drawn, has four.
- “Three, one to each of three other carbon atoms.” — Some edge atoms show three solid bonds and a dashed stub. Three bonds per atom, in flat layers, is graphite; in diamond each atom uses all four outer electrons in four bonds.
- “Four, all to carbon atoms in one flat layer.” — Diamond has no layers. Its four bonds point to the corners of a tetrahedron, so the network goes on in all three directions.
- “Only diamond is a giant covalent structure.” — Graphite is too: covalent bonds run through each whole layer, and there are no separate molecules.
- “Diamond has delocalised electrons too.” — Diamond uses all four outer electrons of each atom in bonds, so it has none.
- “Graphite is soft because its covalent bonds are weak.” — The covalent bonds inside each layer are strong. The forces between the layers are weak, so the layers slide over each other.
- “Graphite is slippery because of its delocalised electrons.” — The delocalised electrons explain why graphite conducts. It is slippery because its layers slide over each other.
- “Graphite is made of small round molecules that roll like ball bearings.” — Graphite has no separate molecules. Each layer is one giant covalent structure, and whole layers slide.
- “Graphite conducts because it contains ions.” — Graphite has no ions. Its delocalised electrons, one from each carbon atom, carry the current.
Scope
Giant covalent and metallic structures (2.6 Core 1–2, Supplement 3–4; 2.7 Supplement 1–2): the giant covalent structures of diamond and graphite and their uses in cutting tools, as a lubricant and as an electrode, the giant covalent structure of silicon(IV) oxide and its similarity in properties to diamond, metallic bonding as the attraction between positive ions in a giant metallic lattice and a sea of delocalised electrons, and the electrical conductivity, malleability and ductility of metals. Simple molecules, ionic lattices, alloys, the uses of metals and electrolysis are left to other modules.