Connect Period 3 reactions with the structures and bonding that explain the properties of their oxides.
Start with the reacting species
This topic extends Year 12 periodicity. The selected elements are Na, Mg, Al, Si, P and S; chlorine and argon oxides are not part of this comparison. Moving across the period changes the stable oxide structures from predominantly ionic lattices to covalent networks and molecules. That change explains both physical properties and acid–base behaviour, but these are two different arguments.
For an oxidation state, assign oxygen −2 in these oxides and make the total equal the species charge. Thus phosphorus is +5 in P₄O₁₀ and sulfur is +4 in SO₂ but +6 in SO₃. The highest oxide in the specified sulfur comparison is SO₃, even though sulfur burns mainly to SO₂.
Sodium and magnesium with water
Sodium reacts vigorously with cold water: a moving piece gets smaller, hydrogen is evolved and the remaining solution is alkaline. Magnesium reacts much more slowly with cold water because its surface layer restricts reaction; a little hydrogen and sparingly soluble magnesium hydroxide form. With steam, heated magnesium produces magnesium oxide and hydrogen. The white MgO is a solid formed in a gas–solid reaction, not a precipitate from solution.
Building the required oxides
Use the following balanced equations for the AQA products. Magnesium burns with an intense white light; aluminium powder can burn after the protective oxide layer is overcome; silicon requires strong heating. Phosphorus burns readily and sulfur burns with a blue flame. White solids are obtained for Na₂O, MgO, Al₂O₃, SiO₂ and P₄O₁₀; SO₂ is a colourless gas. Avoid directly smelling products.
Sodium can also form peroxide under some conditions; Na₂O is the oxide specified here. SO₃ is prepared by further oxidation of SO₂, using a catalyst in the Contact process, rather than by assuming direct combustion of sulfur produces only SO₃.
Identify what is held together
| Oxide | Structure and bonding | What must be overcome on melting? |
|---|---|---|
| Na₂O | Giant ionic lattice: Na⁺ and O²⁻ | Electrostatic attractions between oppositely charged ions |
| MgO | Giant ionic lattice: Mg²⁺ and O²⁻ | Stronger ionic attractions than in Na₂O, linked to charge and size |
| Al₂O₃ | Giant lattice with substantial ionic bonding and some covalent character | Strong attractions throughout the solid; not a simple molecular oxide |
| SiO₂ | Giant covalent network | Many strong Si–O covalent bonds |
| P₄O₁₀ | Common molecular form contains discrete P₄O₁₀ units | Intermolecular attractions; other solid forms exist |
| SO₂ and SO₃ | Simple molecular treatment for this comparison | Intermolecular attractions; SO₃ has more complicated solid forms |
Explain the broad pattern without inventing a smooth trend
Na₂O, MgO and Al₂O₃ have high melting points. MgO exceeds Na₂O because Mg²⁺ has greater charge and a smaller radius than Na⁺, giving stronger attractions to oxide ions. Do not extrapolate this into a compulsory rise from MgO to Al₂O₃: real structures and covalent character matter, and MgO has the higher melting point.
SiO₂ also melts only at high temperature, but the explanation is a covalent network, not an ionic lattice. The marked fall to the molecular oxides occurs because melting mainly overcomes intermolecular attractions, not the strong covalent bonds within each molecule. Among molecular oxides, size, polarisability, molecular arrangement and the solid form matter. Use any supplied melting data rather than asserting a universal perfectly ordered sequence.
Diagram placeholder
Oxide structure comparison to add
Labels to include:
- Na⁺ and O²⁻ in a repeating ionic lattice
- Each Si joined to four O; each bridging O joined to two Si
- Separate P₄O₁₀ molecular units
- Strong bonds inside units; weaker attractions between units
Show a lattice, a continuing Si–O network and separated molecular units in three panels. The SiO₂ network must not appear as isolated O=Si=O molecules. Mark the interactions overcome on melting in each panel.
Quick checks
Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official AQA mark allocations.
Q1. Balance the formation of aluminium oxide from its elements.Show answer
4Al(s) + 3O₂(g) → 2Al₂O₃(s). Four Al atoms and six O atoms appear on each side.
Q2. Why does magnesium give different products with cold water and steam?Show answer
Under cold-water conditions it forms Mg(OH)₂ slowly. With steam, the balanced reaction is Mg + H₂O → MgO + H₂. State the conditions; do not write Mg(OH)₂ as the steam product.
Q3. Explain the large fall in melting point from SiO₂ to molecular P₄O₁₀.Show answer
SiO₂ is a giant covalent network requiring many strong covalent bonds to be broken. Molecular P₄O₁₀ melts by overcoming weaker intermolecular attractions, so less energy is required.
Q4. A learner says Al₂O₃ must melt above MgO because Al³⁺ has the larger charge. Evaluate the argument.Show answer
Ion charge alone does not determine the melting point. Structure, ion size and covalent character also matter; MgO in fact melts above Al₂O₃. The simple charge argument supports a comparison of MgO with Na₂O more reliably.
Q5. Find sulfur’s oxidation state in SO₂ and SO₃, and explain how SO₃ is obtained.Show answer
Oxygen is −2. Sulfur is +4 in SO₂ and +6 in SO₃. Sulfur combustion primarily gives SO₂; catalytic oxidation, 2SO₂ + O₂ ⇌ 2SO₃, supplies SO₃.
Sources
Sources and examiner guidance (reviewed 2 October 2026)
- AQA 7405 inorganic chemistry specification — 3.2.4 coverage and required skills.
- Chemrevise: Properties of Period 3 elements and their oxides — Coverage checklist; explanations, data exercises and quick checks on this page are original Finesse material.
- AQA June 2023 Paper 1 mark scheme — Q03.1–03.5, pp14–15: combustion, sulfur dioxide in water, oxyanion structure, steam and acid–oxide stoichiometry; paired with report Q03 p3.
- AQA June 2023 Paper 1 examiner report — Read alongside the question-specific marking guidance; not a universal wording checklist.
Finesse Tuition is not endorsed by AQA or Chemrevise. All explanations and examples here are our own.