Explain why the melting points of Period 3 elements first increase and then decrease across the period, from sodium to argon.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The melting points of Period 3 elements vary considerably. Sodium has a melting point of 98 °C, silicon has a melting point of 1414 °C, and argon has a melting point of −189 °C.
Model answer (5 marks)
Sodium, magnesium and aluminium are metals with metallic bonding in giant structures, so their melting points rise from Na to Al.
The number of delocalised electrons per atom increases across the period, giving stronger metallic bonds and higher melting points.
Silicon has a giant covalent (network) structure, giving it the highest melting point.
Phosphorus, sulphur and chlorine are simple covalent molecules, and argon is a noble gas; all have weak intermolecular forces, so their melting points are low.
Argon, as individual atoms, has the weakest forces and the lowest melting point.
The number of delocalised electrons per atom increases across the period, giving stronger metallic bonds and higher melting points.
Silicon has a giant covalent (network) structure, giving it the highest melting point.
Phosphorus, sulphur and chlorine are simple covalent molecules, and argon is a noble gas; all have weak intermolecular forces, so their melting points are low.
Argon, as individual atoms, has the weakest forces and the lowest melting point.
Examiner tips
- Use the word ‘metallic bonding’ for Na–Al, ‘giant covalent’ for Si, and ‘simple covalent’ or ‘intermolecular’ for P, S, Cl, Ar.
- Show the trend: increase (Na→Al) then decrease (Al→Ar).
- Mention delocalised electrons for metals and network structure for Si.
- Explain argon’s lone‑atom nature and weak London forces.
Common mistakes
- Confusing the order of elements (e.g., saying melting points rise to Si then fall to Ar without noting the intermediate metals).
- Failing to mention the type of bonding for each element (metallic, covalent, or intermolecular).
- Overlooking that argon’s low melting point is due to weak London dispersion forces between atoms.
Mark scheme (5 marks)
- Sodium, magnesium and aluminium are metals with metallic bonding / giant metallic structures, so melting points increase from Na to Al
- More delocalised electrons / stronger metallic bonding as the number of delocalised electrons per atom increases from Na to Al, so more energy is needed to overcome these forces
- Silicon has the highest melting point because it has a giant covalent / giant atomic structure
- Phosphorus, sulfur, chlorine and argon are simple molecular / simple covalent structures, so they have low melting points
- Argon (noble gas) exists as individual atoms with very weak intermolecular forces, so it has the lowest melting point
Related
- All AQA A-Level Chemistry (7405) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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