Exam Frequency Analysis
Past paper frequency (2018 to 2024)
This topic accounts for approximately 8% of your exam marks.
stable
Low
Stable8%
Dot-and-cross diagrams and comparison of giant covalent vs simple molecular structures appear regularly.
What they are
- A (also called a macromolecule) contains a vast number of non-metal atoms joined by strong covalent bonds in one continuous lattice
- Two classic examples, both built from carbon only:
- Diamond and graphite are of carbon: different forms of the same element with different bonding arrangements
Why all giant covalent structures have high melting points
- There are many strong running throughout the lattice
- Melting or vaporising the solid requires breaking very large numbers of these bonds
- That takes a great deal of thermal energy, so the melting point is high
Diamond
- Each carbon atom forms 4 single covalent bonds to four other carbon atoms
- The result is a rigid 3D tetrahedral network extending throughout the crystal
- Every outer electron on every carbon is locked into a covalent bond → no free electrons
Properties of diamond:
- Very hard: the rigid 3D network of strong covalent bonds resists deformation. Diamond is used in cutting tools, drill bits and saw edges where extreme hardness matters
- High melting point: the many strong covalent bonds running through the lattice need a large amount of energy to break
- Does not conduct electricity: every outer electron is locked in a covalent bond, so there are no free electrons or ions to move and carry charge
Exam tip
Explain why diamond has a high melting point (3 marks)
Write (three marks): (1) diamond has a giant covalent structure — a continuous 3D lattice; (2) it contains many strong covalent bonds throughout the lattice that must be broken to melt it; (3) breaking so many strong bonds requires a very large amount of energy.
Watch out: never refer to intermolecular forces or ions in diamond. The mark scheme awards no marks for M2 or M3 if you mention intermolecular forces — diamond has no intermolecular forces, only covalent bonds.
Graphite
- Each carbon atom forms only 3 single with three other carbon atoms
- These bonded atoms arrange themselves into flat layers of hexagons (sometimes called sheets)
- Each carbon has 4 outer electrons in total; 3 are used in bonds and the 4th is a that can move freely along the layer
- The hexagonal layers stack on top of each other, held together only by weak between layers, not by covalent bonds
Properties of :
- Soft and slippery: the weak forces between layers let the layers slide easily over each other. Graphite is used as a dry lubricant and as the writing "lead" in pencils
Exam tip
Explain why graphite conducts electricity (2 marks)
Write (two marks): (1) graphite has delocalised electrons; (2) those electrons are free to move throughout the structure.
Watch out: M2 depends on M1 — you must name electrons before saying they move. Saying "ions move" or "atoms move" scores zero for the whole question. "Carries charge" on its own is not enough for M2; the mark scheme requires you to say the electrons can move or flow.
Exam tip
Why diamond is hard but graphite is soft
What comes up: explain the difference in hardness between diamond and graphite.
Watch out: graphite's softness comes from layers sliding over each other because the forces between the layers are weak — not from "weak bonds". The covalent bonding within each layer (and throughout diamond's 3D lattice) stays strong.