Diamond -- four bonds, rigid 3D lattice
A giant covalent (macromolecular) structure is a huge number of atoms joined by strong covalent bonds throughout one repeating lattice -- effectively a single enormous molecule, with no separate small molecules and no weak intermolecular forces inside it. In diamond every carbon atom forms four covalent bonds to four other carbon atoms, arranged tetrahedrally in three dimensions, and that pattern repeats through the whole crystal.
Graphite -- three bonds, layers, spare electron
Graphite is carbon arranged in flat hexagonal rings joined into layers. Each carbon atom forms only three covalent bonds within its layer, so one outer electron per atom is left over and becomes delocalised -- fixed to no atom or bond, and free to move along the layer. Between the layers there are only weak intermolecular forces, not covalent bonds, so the layers slide over each other easily.
(Extended) Structure explains every use
Give the structural reason, never just the property. Diamond: four strong covalent bonds per atom in a rigid 3D lattice, bonds in every direction, no plane to slip along -> very hard -> cutting tools and drill tips. Graphite as lubricant: weak forces between layers let them slide -> less friction. Graphite as electrode: delocalised electrons free to move carry charge -> it conducts, and its very high melting point survives electrolysis.
Drawn from real examiner reports.
Diamond is hard, not strong or tough
Examiners want hard -- resistant to scratching and indentation -- linked to diamond's structure. "Strong" and "tough" are different material properties, not the tested term. The chain is: four strong covalent bonds per carbon atom, in a rigid 3D lattice with bonds in every direction, so there is no plane to wear away -- so diamond is hard, so it suits cutting tools.
Flagged Jun 2022 P43 Q11ei; Nov 2022 P43 Q11a; Nov 2023 P41 Q8d
Conduction is not a list of differences
When a question asks specifically why graphite conducts electricity, the mark needs delocalised electrons -- one per carbon atom, left over because each atom forms only three bonds -- that are free to move and carry charge. Candidates instead list general differences ("graphite is soft, diamond is hard"), which never addresses conduction and scores nothing.
Flagged Jun 2022 P43 Q11ei; Nov 2022 P43 Q11a; Nov 2023 P41 Q8d
Giant covalent is not giant ionic
A giant covalent structure has no ions. Diamond and graphite are neutral atoms sharing pairs of electrons. A giant ionic lattice such as sodium chloride is held by electrostatic attraction between oppositely charged ions, with no covalent bonds. Both have high melting points, but quoting the ionic reason for diamond, or the covalent reason for salt, loses the mark.
Giant covalent is not simple molecular
A giant covalent structure has no separate small molecules: strong covalent bonds run through the crystal. Simple molecular substances such as water and carbon dioxide melt at low temperatures because only weak intermolecular forces are overcome, not covalent bonds. Say which is being broken -- confusing covalent bonds with intermolecular forces is the flagged error.
Flagged Jun 2023 P42 Q8e
Hardness is not high melting point
These have different causes. Diamond is hard because its rigid 3D lattice offers no slip plane. The high melting point of both diamond and graphite comes from breaking a very large number of strong covalent bonds throughout the structure. Graphite is soft (its layers slide) yet still melts at a very high temperature -- so never argue that soft means weakly bonded.
No bonds between graphite layers
Because the layers are held only by weak forces, a graphite diagram must show a clear gap with no bonds between layers -- drawing covalent bonds from one layer to the next contradicts the sliding explanation. Keep the bond counts straight: three bonds per atom in graphite, in flat hexagonal rings, and four in diamond, in a 3D lattice that is never drawn flat.
Answer the one property asked
These questions ask about ONE property, not a general comparison. Identify which it is -- conduction, hardness, lubrication or melting point -- then name the single structural feature that causes it, and stop there.
(Extended) Link structure, property, use
Write the link out with "so" or "because": structural feature -> property -> use. A property stated with no structural cause, or a named use with no property, drops the linking mark on "relate the structure and bonding to the use" questions.
Use the mark-scheme words
Particular words earn the marks: delocalised and free to move for conduction, hard for diamond, weak forces and slide for the lubricant, giant/macromolecular for the structure itself. Near-misses such as "loose electrons" or "strong" usually score nothing.
Cambridge 0654 spec reference: Section C2 "Atoms, elements and compounds", sub-topic C2.6 "Giant covalent structures". Core: describe the giant covalent structures of graphite and diamond. Extended: relate the structure and bonding of graphite (lubricant, electrode) and diamond (cutting tools) to their uses. This leaf covers only graphite and diamond -- silicon dioxide is not a taught example in the current syllabus.
| Term | Mark-scheme-precise meaning |
|---|---|
| Giant covalent structure (macromolecular structure) | A huge number of atoms joined together by strong covalent bonds throughout a giant, repeating lattice -- effectively one giant molecule, with no weak intermolecular forces between separate small molecules |
| Covalent bond | A shared pair of electrons between two atoms |
| Delocalised electron | An electron that is not fixed to a single atom or bond and is free to move through the whole structure |
Diamond -- structure and bonding:
Graphite -- structure and bonding:
Properties -> uses (Extended):
| Substance | Key structural feature | Property | Use |
|---|---|---|---|
| Diamond | Rigid 3D lattice, 4 strong covalent bonds per atom, every direction | Very hard | Cutting tools / drill tips |
| Graphite | Weak forces between flat layers | Layers slide easily | Lubricant |
| Graphite | Delocalised electrons free to move within a layer | Conducts electricity; also has a very high melting point | Electrode (e.g. in electrolysis) |
A note on moles and giant covalent structures (Extended stoichiometry link): diamond and graphite are both forms of the element carbon, so the relative atomic mass () of carbon, 12, is also the relative formula mass () of the substance. This lets you calculate the number of moles of carbon atoms in a given mass of diamond or graphite using , where = amount (mol), = mass (g), = relative formula mass. (This mole calculation is Extended/Supplement content -- the Core tier does not use the mole concept.)
Define a delocalised electron.
Diamond and graphite are both giant covalent structures made entirely of the element carbon.
Calculate the relative formula mass () of carbon. (1 mark)
