Cognito · AQA ChemistryCovalent structures and properties
Key facts
Covalent bonds
Formed between non-metals by sharing pairs of electrons.
Each shared pair = one covalent bond.
Atoms share electrons to achieve a full outer shell (like noble gas configuration).
Examples: H₂ (single bond), O₂ (double bond), N₂ (triple bond), H₂O (2 single bonds), CH₄ (4 single bonds).
Dot-and-cross diagrams show only outer shell electrons.
Simple molecular structures
Small molecules held together by strong covalent bonds.
Between molecules: weak intermolecular forces.
Properties: low melting/boiling points (weak forces between molecules), does not conduct electricity (no ions or free electrons).
Examples: H₂O, CO₂, CH₄, NH₃, HCl.
The covalent bonds themselves are NOT broken on melting — only the weak forces between molecules.
Giant covalent structures
Diamond: each C bonded to 4 others in tetrahedral arrangement. Very hard, very high melting point, does not conduct electricity.
Graphite: each C bonded to 3 others in layers. Layers can slide → lubricant. Has delocalised electrons → conducts electricity.
Silicon dioxide (SiO₂): giant network, very hard, high melting point, no conduction.
Graphene: single layer of graphite. Conducts electricity. Very strong.
Polymers
Large molecules made of many repeating units (monomers) joined by covalent bonds.
Thermosoftening polymers: weak intermolecular forces between chains → melt on heating.
Thermosetting polymers: cross-links between chains → do not melt.
Properties depend on chain length, branching, and cross-linking.
Exam questions — 4 questions · 13 marks
⚡ This module includes extended questions. Read the hint. Write in full sentences. The AI marks against the real AQA mark scheme.
1 markCovalent bond definitionAQA 8462 P1 style
Which of the following correctly describes a covalent bond?
A Transfer of electrons from a metal to a non-metal forming ions
B Electrostatic attraction between oppositely charged ions
C Sharing of a pair of electrons between two non-metal atoms
D The attraction between a positive metal ion and delocalised electrons
1 markGraphite conductivityAQA 8462 P1 style
Why does graphite conduct electricity but diamond does not?
A Graphite has ionic bonds; diamond has covalent bonds
B Graphite has delocalised electrons free to move between layers; diamond has all electrons fixed in covalent bonds
C Graphite has a higher melting point giving electrons more energy to move
D Diamond is an insulator because it has no outer electrons
4 marksExplain properties of diamondAQA 8462 P1 style
Explain why diamond has a very high melting point and does not conduct electricity. (4 marks)
Hint: Cover: structure (giant covalent) → why melting point is high (strong bonds, lots of energy to break) → why no conduction (no free electrons/ions).
+40 XP
4 marksCompare diamond and graphiteAQA 8462 P1 style
Explain why graphite is used as a lubricant and as an electrode. Refer to the structure and bonding of graphite. (4 marks)