Explain why metals have high melting points and why the melting point of a metal generally increases across Period 3 from sodium to aluminium.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Metals consist of a lattice of positive ions surrounded by a sea of delocalised electrons. The metallic bond is the electrostatic attraction between the ions and this electron sea. To melt a metal, energy must be supplied to overcome these attractive forces, so metals generally have high melting points.
Across Period 3, the number of delocalised electrons per atom increases from Na (1) to Mg (2) to Al (3). The greater number of electrons (and the higher ionic charge) strengthens the electrostatic attraction between the ions and the electron sea, requiring more energy to separate the ions. Consequently the melting point rises from Na to Al.
Across Period 3, the number of delocalised electrons per atom increases from Na (1) to Mg (2) to Al (3). The greater number of electrons (and the higher ionic charge) strengthens the electrostatic attraction between the ions and the electron sea, requiring more energy to separate the ions. Consequently the melting point rises from Na to Al.
Examiner tips
- Use the term ‘metallic bond’ and ‘delocalised electrons’.
- Explain that melting requires overcoming electrostatic attraction.
- Show the trend in electron number from Na to Al.
- Mention the higher ionic charge for Al.”]
Mark scheme (4 marks)
- In metals, positive ions (cations/kernels) are surrounded by a 'sea' of delocalised electrons, and the metallic bond is the electrostatic attraction between these positive ions and the delocalised electrons.
- To melt a metal, sufficient energy must be supplied to overcome these electrostatic attractive forces, so metals generally have high melting points.
- Going from Na to Al across Period 3, the number of delocalised electrons per atom increases (Na provides 1, Mg provides 2, Al provides 3).
- More delocalised electrons (and a higher ionic charge) result in stronger electrostatic attraction between the ions and the electron sea, so more energy is required to separate the ions and the melting point increases.
Key terms in this question
Related
- All IB DP Chemistry Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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