Choose the particles and forces first. Use metallic, giant covalent and molecular structures to explain the uneven melting-point pattern across Period 3.
Choose the correct model before explaining a melting point
There is no single bonding explanation that works across Na–Ar. Split the period by structure. Heating a molecular substance changes the arrangement of its molecules; it does not normally separate their atoms. Heating a giant structure requires disruption of the bonding throughout its extended network.
| Elements | Structure | Interaction overcome on melting |
|---|---|---|
| Na, Mg, Al | Giant metallic lattice | Attraction between positive metal ions and delocalised electrons |
| Si | Giant covalent network | Strong covalent bonds between silicon atoms |
| P₄ (white phosphorus), S₈, Cl₂ | Simple molecular | London forces between molecules |
| Ar | Monatomic | London forces between atoms |
Na, Mg and Al: explain metallic attraction
The melting point rises from Na to Mg and then to Al. In the simple metallic model, each atom supplies respectively one, two or three delocalised electrons and forms a positive metal ion. Increasing ion charge, decreasing ion size and more delocalised electrons produce stronger electrostatic attraction. More energy is needed to disrupt the metallic lattice.
Write “attraction between positive metal ions and delocalised electrons”. Do not use “intermolecular forces” for a metal.
Why silicon forms the high point
Silicon atoms form an extended covalent network; each is covalently bonded to four others. Many strong covalent bonds must be broken to melt this structure, requiring much energy. Silicon therefore has a much higher melting point than the molecular elements that follow.
Do not call silicon a molecule or say that it has strong intermolecular forces. “Giant” describes the continuous network, not an unusually large individual atom.
Why P₄, S₈, Cl₂ and Ar are much lower
White phosphorus contains P₄ molecules and sulfur contains S₈ molecules. Chlorine is Cl₂, while argon consists of individual atoms. London attractions between these particles require less energy to overcome than the extensive bonding in silicon.
For these molecular examples the usual order is S₈ above P₄ above Cl₂. An S₈ molecule contains 128 electrons, P₄ has 60 and Cl₂ has 34. Larger, more polarisable electron clouds allow stronger instantaneous-dipole–induced-dipole attractions. More energy is needed to separate the molecules.
Argon has only 18 electrons per atom and very weak interatomic London forces, giving a very low melting point. Use “atoms”, not “molecules”, for Ar.
The P₄ comparison refers to white phosphorus. Other allotropes have different structures and properties. Melting points also depend on crystal packing, so electron count is not a universal way of ranking unrelated substances.
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Period 3 structure-to-melting-point overview
Labels to include:
- Na–Ar along the horizontal axis
- Vertical axis: melting point / K
- Metallic region, silicon network, P₄/S₈/Cl₂ molecular region, monatomic Ar
- Peak at Si; S₈ above P₄; Cl₂ above Ar
- Separate sketches of metal ions/electrons, a Si network and discrete molecules
The dramatic drop from Si to P comes from a change of structure and the interaction overcome, not a sudden weakening of covalent bonds inside P₄.
A repeatable comparison method
1. Name both structures. 2. Identify the particles and the bonds or forces involved. 3. Compare their strength, using a relevant particle-level reason. 4. Link that to the energy required and the melting-point difference.
Example: sulfur versus silicon. Sulfur consists of S₈ molecules with London attractions between them. Silicon is a giant covalent network. Overcoming the intermolecular attractions in sulfur requires less energy than breaking many covalent bonds in silicon, so sulfur melts at a lower temperature.
Do not equate high melting point with high reactivity. Melting is a physical change; reactivity concerns a chemical process. Sodium is reactive but melts much more easily than silicon.
Quick checks
Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official AQA mark allocations.
Q1. Why is “sulfur has weaker covalent bonds than silicon” a poor melting-point explanation?Show answer
It identifies the wrong interaction for sulfur. Melting sulfur overcomes London forces between S₈ molecules, whereas melting silicon disrupts its giant covalent network.
Q2. Give a complete reason why aluminium melts at a higher temperature than sodium.Show answer
Al has smaller, more highly charged positive ions and more delocalised electrons per atom. This gives stronger metallic attraction, so more energy is needed to disrupt the lattice.
Q3. Count the electrons in P₄ and S₈ using proton numbers 15 and 16. Explain the usual melting-point order.Show answer
P₄: 4 × 15 = 60; S₈: 8 × 16 = 128. S₈ has the larger, more polarisable electron cloud and stronger London attractions between molecules. More energy is needed, so sulfur has the higher melting point.
Q4. Which particles should appear in an explanation of argon melting?Show answer
Individual Ar atoms and London forces between them. Argon is monatomic, so “Ar molecules” is incorrect.
Q5. A substance has a very high melting point but does not conduct as a solid like a metal. Is this enough to prove it is silicon?Show answer
No. It is consistent with a giant covalent structure but does not uniquely identify an element. Use the other evidence in the question; high melting point alone is not an identity test.
Sources
Sources and examiner guidance (reviewed 2 October 2026)
- Chemrevise — AQA 2.1 Periodicity — Coverage checklist; original explanations and practice below.
- AQA 7405 specification — 3.2.1.1–3.2.1.2.
- AQA 7404/1 2020 mark scheme (November archive) — Q01.1, p11; Q05.1, p16.
- AQA 7404/1 June 2019 mark scheme — Q04, p9: Period 2 comparison.
- AQA 7404/1 June 2019 examiner report — Q04, pp4–5: use the correct period and explain both deviations.
Finesse Tuition is not endorsed by AQA or Chemrevise. All explanations and examples here are our own.