Explain why the melting point of tungsten (W) is significantly higher than that of calcium (Ca), even though both metals are described using the metallic bonding model.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Tungsten has a melting point of 3422 °C and belongs to Period 6, Group 6. Calcium has a melting point of 842 °C and belongs to Period 4, Group 2.
Model answer (4 marks)
Both metals are lattices of cations surrounded by a sea of delocalised electrons, so melting requires breaking the electrostatic attraction between the ions and the electron sea.
1. Tungsten can donate up to 6 valence electrons to the electron sea, whereas calcium only donates 2, giving tungsten a higher electron density.
2. Tungsten ions have a higher effective nuclear charge (high oxidation states) than Ca²⁺, so the attraction between the cations and the electron sea is stronger.
3. The combination of a higher ionic charge and a larger number of delocalised electrons makes the metallic bonds in tungsten much stronger.
4. Consequently, more energy is required to overcome these bonds, giving tungsten a much higher melting point than calcium.
1. Tungsten can donate up to 6 valence electrons to the electron sea, whereas calcium only donates 2, giving tungsten a higher electron density.
2. Tungsten ions have a higher effective nuclear charge (high oxidation states) than Ca²⁺, so the attraction between the cations and the electron sea is stronger.
3. The combination of a higher ionic charge and a larger number of delocalised electrons makes the metallic bonds in tungsten much stronger.
4. Consequently, more energy is required to overcome these bonds, giving tungsten a much higher melting point than calcium.
Examiner tips
- Use the phrase "electrostatic attraction between the cations and the electron sea" to show understanding of metallic bonding.
- Mention both the higher number of delocalised electrons and the higher ionic charge to hit all four points.
- Keep the answer concise – 4 points can be covered in 4 short sentences or bullet points.
Common mistakes
- Confusing the role of valence electrons with the number of electrons in the electron sea.
- Forgetting that the ionic charge of W is higher than Ca²⁺.
- Writing a long paragraph instead of clear, separate points that match the mark scheme.
Mark scheme (4 marks)
- Both metals consist of a lattice of positive ions (cations) surrounded by a 'sea' of delocalised electrons, and melting requires overcoming electrostatic attraction between the ions and delocalised electrons.
- Tungsten contributes more delocalised electrons per atom than calcium (up to 6 valence electrons available for delocalisation versus 2 for calcium), resulting in a higher charge density of the electron sea.
- Tungsten ions carry a higher ionic charge (W can exhibit high oxidation states, effectively contributing a higher nuclear/ionic charge) than Ca²⁺, increasing the electrostatic attraction between the cations and the electron sea.
- The combined effect of higher ionic charge and greater number of delocalised electrons makes the metallic bonds in tungsten much stronger, requiring significantly more energy to overcome, hence the much higher melting point.
Key terms in this question
metallic bonding · melting point
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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