Explain why metals are generally malleable and ductile, whereas ionic compounds are brittle.

IB DP Chemistry Higher Level (2023 syllabus) — S2.3 The metallic model · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Metals have a lattice of cations surrounded by a delocalised sea of electrons. When a shear force is applied, the cation layers can slide over one another because the electrons move with the layers and continue to attract the cations, so the structure is not broken. This gives malleability and ductility.

Ionic compounds consist of a rigid lattice of alternating positive and negative ions held by strong electrostatic forces. A stress causes adjacent layers to shift so that like‑charged ions come into contact. The resulting repulsion between like charges breaks the lattice, making the crystal brittle.

Examiner tips

  • Use the terms ‘delocalised electrons’ and ‘electrostatic attraction’ for metals; mention ‘rigid lattice’ and ‘like‑charge repulsion’ for ionic compounds.
  • Show the cause (electron sea vs fixed ions) and effect (sliding vs breaking) clearly.
  • Keep the answer concise – 4 marks only.
  • Use UK spelling (e.g., ‘behaviour’).

Common mistakes

  • Confusing metallic bonding with covalent or ionic bonding; not mentioning the electron sea.
  • Describing ionic compounds as ‘metallic’ or saying they are ductile.
  • Using vague terms like ‘strong forces’ without specifying electrostatic attraction or repulsion.

Mark scheme (4 marks)

  1. In metals, positive ions (cations/kernels) are surrounded by a 'sea' of delocalised electrons
  2. When a force is applied, layers of metal cations can slide/move past one another without disrupting the bonding, because the delocalised electrons move with the layers and maintain electrostatic attraction throughout
  3. In ionic compounds, the lattice contains alternating oppositely charged ions held by strong electrostatic forces in fixed positions
  4. When a stress/force is applied to an ionic compound, layers shift so that like charges align, causing strong repulsion between adjacent ions of the same charge, which shatters/cleaves the lattice

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