Explain why the melting points of the Group 1 metals decrease down the group from lithium to caesium.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Metallic bonding is the electrostatic attraction between delocalised electrons and the positive metal ions.
As you go down Group 1 the ionic radius increases, so the distance between the nucleus and the delocalised electrons becomes larger.
The larger ions also contain more inner‑shell electrons, giving greater shielding and reducing the effective nuclear charge that the delocalised electrons feel.
Consequently the metallic bond weakens down the group, requiring less energy to break it, and the melting point falls.
As you go down Group 1 the ionic radius increases, so the distance between the nucleus and the delocalised electrons becomes larger.
The larger ions also contain more inner‑shell electrons, giving greater shielding and reducing the effective nuclear charge that the delocalised electrons feel.
Consequently the metallic bond weakens down the group, requiring less energy to break it, and the melting point falls.
Examiner tips
- Use the word ‘metallic bonding’ and ‘delocalised electrons’ early. Explain the size/ionic‑radius trend and its effect on electron–ion attraction. Show the link between weaker bond and lower melting point. Keep the answer concise and use the exact terminology from the mark scheme.
Common mistakes
- Confusing ionic radius with atomic radius. Forgetting that the trend is due to increased shielding and distance. Using vague terms like ‘weaker bond’ without linking to melting point.
Mark scheme (4 marks)
- Metallic bonding involves electrostatic attraction between delocalised electrons and positive metal ions (cations).
- Going down Group 1, the ionic radius increases, so the distance between the nucleus and the delocalised electrons increases.
- The larger ions also have more shielding from inner electron shells, reducing the effective nuclear charge experienced by the delocalised electrons.
- As a result, the metallic bond becomes weaker down the group, so less energy is required to overcome it, and the melting point decreases.
Related
- All IB DP Chemistry Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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