Ionic bonding and properties — extended response

Question 5 (6 marks)

Describe the structure of an ionic compound and use this to explain its physical properties: high melting point, electrical conductivity and brittleness. (WJEC 6-mark extended response)

Mark scheme (AO1 + AO2, 6 marks):

• Structure: giant ionic lattice / 3D regular arrangement of alternating positive and negative ions; held together by strong electrostatic forces in all directions (1–2 marks)
• High melting point: strong electrostatic forces throughout the lattice require large amounts of energy to overcome → high temperature needed to melt (1 mark)
• Electrical conductivity: solid — ions in fixed positions → cannot conduct; when molten or dissolved → ions free to move → can carry charge → conduct electricity (1–2 marks)
• Brittleness: hard because ions are held rigidly; when a force is applied, layers of ions shift → like charges align next to each other → electrostatic repulsion → lattice shatters along a plane (1 mark)