HookThe fog that killed 4,000 Londoners was an oxide of Period 3

For five days in December 1952, a windless anticyclone parked itself over London and the smoke of a million coal fires had nowhere to go. Visibility fell to a metre in places; conductors walked in front of their own buses with flares. When the Great Smog lifted, the initial death toll was put at around 4,000 — later analyses pushed it towards 12,000. The killer was not the soot you could see but a gas you could not: sulfur dioxide, dissolving in fog droplets to form sulfurous acid and carrying acid deep into people's lungs. Parliament answered with the Clean Air Act 1956.

Sulfur sits at the right-hand end of Period 3. Its oxide turns water into acid. Walk to the left-hand end of the same row and sodium's oxide does the opposite — drop it in water and you get sodium hydroxide at pH 14, an alkali strong enough to dissolve grease and skin. In between sits aluminium oxide, which shrugs at water entirely, and silicon dioxide, which is literally the stuff of mountains. Section 3.2.4 asks one question: how does bonding change across a row, and what does that do to the chemistry of the oxides? Master the pattern — ionic and basic on the left, macromolecular in the middle, molecular and acidic on the right — and every equation in this section writes itself.

ModelSodium and magnesium meet water — same period, different tempo

Sodium reacts vigorously with cold water: the metal melts into a shivering silver ball, skates across the surface on a cushion of hydrogen, and leaves a strongly alkaline solution. The equation is \(2\text{Na} + 2\text{H}_2\text{O} \rightarrow 2\text{NaOH} + \text{H}_2\), and because sodium hydroxide is freely soluble the resulting solution sits at around pH 13–14.

Magnesium, one place to the right, barely reacts with cold water at all — a strip left in a beaker for days grows only a few bubbles. The reaction \(\text{Mg} + 2\text{H}_2\text{O} \rightarrow \text{Mg(OH)}_2 + \text{H}_2\) is extremely slow, and the magnesium hydroxide it makes is only sparingly soluble, so the solution is weakly alkaline, around pH 9–10. That phrase 'sparingly soluble' is the exact wording AQA credits: the solubility of the hydroxide, not just the speed of the reaction, is why the pH is lower than sodium's.

Heat magnesium in steam, though, and the tempo changes completely: it burns with a brilliant white flame to give the oxide, not the hydroxide — \(\text{Mg} + \text{H}_2\text{O(g)} \rightarrow \text{MgO} + \text{H}_2\). Two reasons sit behind the contrast with sodium: magnesium's two outer electrons are removed against a higher total ionisation energy, and the Mg(OH)\(_2\) crust that forms in cold water blankets the metal surface. One row step, and the reaction goes from 'dangerous in a beaker' to 'needs a Bunsen and a steam generator'.

DataThe oxides — three structures, one melting-point story

Burn the Period 3 elements in oxygen (sodium with a yellow flame to Na\(_2\)O, magnesium blinding white to MgO, sulfur blue to SO\(_2\)) and the oxides you collect fall into three structural families, and the melting points betray which is which.

Giant ionic lattices: Na\(_2\)O, MgO and Al\(_2\)O\(_3\). Melting them means overcoming electrostatic attraction throughout the whole lattice, so the values are enormous — Na\(_2\)O about 1,130 °C, MgO about 2,850 °C, Al\(_2\)O\(_3\) about 2,070 °C. MgO outmelts Na\(_2\)O because Mg\(^{2+}\) carries twice the charge of Na\(^{+}\) and is smaller, so the attraction per ion is stronger — which is exactly why MgO lines furnaces as a refractory brick. Al\(_2\)O\(_3\) breaks the simple trend: the tiny, triply charged Al\(^{3+}\) polarises the oxide ion so strongly that the bonding gains covalent character, and the melting point slips below MgO's.

Macromolecular (giant covalent): SiO\(_2\). Every silicon is covalently bonded to four oxygens in an endless three-dimensional network — quartz, sand, the bones of granite. Melting it means snapping strong covalent bonds, so the melting point is high, about 1,610 °C.

Simple molecular: P\(_4\)O\(_{10}\), SO\(_2\) and SO\(_3\). The covalent bonds inside each molecule are strong, but melting only has to defeat the weak intermolecular forces between molecules — so P\(_4\)O\(_{10}\) sublimes at about 360 °C, SO\(_3\) melts at 17 °C and SO\(_2\) at −75 °C. A five-thousand-degree spread across one row, and every value is explained by structure, not by luck.

MechanismOxides into water — a pH tour from 14 down to 0

Drop each oxide into water and the structural families reappear as an acid-base trend. The ionic oxides are basic: the oxide ion is a powerful proton acceptor. Sodium oxide dissolves exothermically, \(\text{Na}_2\text{O} + \text{H}_2\text{O} \rightarrow 2\text{NaOH}\), giving pH 13–14. Magnesium oxide reacts only slightly, \(\text{MgO} + \text{H}_2\text{O} \rightarrow \text{Mg(OH)}_2\), and because the hydroxide is sparingly soluble the pH stalls at 9–10.

Aluminium oxide and silicon dioxide are both insoluble in water — the water stays at pH 7. Al\(_2\)O\(_3\)'s lattice is too strongly bound for hydration to pay its way, and SiO\(_2\)'s covalent network offers water no purchase at all. Insoluble, note, is not the same as unreactive — both have acid-base chemistry, it just needs a stronger partner than water.

The molecular oxides are acidic. Phosphorus(V) oxide reacts violently, \(\text{P}_4\text{O}_{10} + 6\text{H}_2\text{O} \rightarrow 4\text{H}_3\text{PO}_4\), giving phosphoric(V) acid at around pH 0–1. Sulfur dioxide gives sulfurous acid, \(\text{SO}_2 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{SO}_3\), around pH 2–3 — the Great Smog reaction, running in fog droplets and lungs. Sulfur trioxide reacts almost explosively to give sulfuric acid, \(\text{SO}_3 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{SO}_4\), pH 0–1. The rule underneath: metal oxides are basic, non-metal oxides are acidic, and the crossover element — aluminium — sits on the fence.

MechanismAcid meets base — and aluminium plays both sides

Classify an oxide properly and its reactions follow. The basic oxides neutralise acids: \(\text{MgO} + 2\text{HCl} \rightarrow \text{MgCl}_2 + \text{H}_2\text{O}\) and \(\text{Na}_2\text{O} + \text{H}_2\text{SO}_4 \rightarrow \text{Na}_2\text{SO}_4 + \text{H}_2\text{O}\). The acidic oxides neutralise bases: \(\text{SO}_2 + 2\text{NaOH} \rightarrow \text{Na}_2\text{SO}_3 + \text{H}_2\text{O}\), and \(\text{P}_4\text{O}_{10} + 12\text{NaOH} \rightarrow 4\text{Na}_3\text{PO}_4 + 6\text{H}_2\text{O}\). Even 'inert' silicon dioxide dissolves in hot concentrated alkali, \(\text{SiO}_2 + 2\text{NaOH} \rightarrow \text{Na}_2\text{SiO}_3 + \text{H}_2\text{O}\) — proof that its water-insolubility was hiding an acidic oxide all along.

Aluminium oxide is amphoteric: it reacts as a base towards acids, \(\text{Al}_2\text{O}_3 + 3\text{H}_2\text{SO}_4 \rightarrow \text{Al}_2(\text{SO}_4)_3 + 3\text{H}_2\text{O}\), and as an acid towards bases, \(\text{Al}_2\text{O}_3 + 2\text{NaOH} + 3\text{H}_2\text{O} \rightarrow 2\text{NaAl(OH)}_4\), forming sodium aluminate. That double life mirrors its position — a metal whose bonding already has one foot in the covalent camp.

This chemistry runs at industrial scale in Britain. Drax power station in North Yorkshire fitted flue-gas desulfurisation in the 1990s: the acidic SO\(_2\) in its exhaust is scrubbed by a slurry of basic calcium compounds — in effect \(\text{CaO} + \text{SO}_2 \rightarrow \text{CaSO}_3\), oxidised onward to calcium sulfate — and the plant sold the gypsum to plasterboard makers. One Period 3 acid-base reaction, several hundred thousand tonnes of acid rain prevented per year.

Worked example

A white solid oxide of a Period 3 element dissolves exothermically in water to give a solution of pH 1. Identify the oxide, write the equation, and give the equation for its reaction with excess sodium hydroxide.

Step 1 — use the pH. pH 1 means a strongly acidic oxide, so it is molecular and from the right of the period: P\(_4\)O\(_{10}\), SO\(_2\) or SO\(_3\). A white solid at room temperature rules out the two sulfur oxides (SO\(_2\) is a gas, SO\(_3\) a liquid on a warm day) — the oxide is \(\text{P}_4\text{O}_{10}\).

Step 2 — the water equation: \(\text{P}_4\text{O}_{10} + 6\text{H}_2\text{O} \rightarrow 4\text{H}_3\text{PO}_4\). Check the balance: 4 P both sides; left oxygen 10 + 6 = 16, right 4 × 4 = 16; hydrogen 12 each side.

Step 3 — excess NaOH neutralises all three protons of phosphoric(V) acid: \(\text{H}_3\text{PO}_4 + 3\text{NaOH} \rightarrow \text{Na}_3\text{PO}_4 + 3\text{H}_2\text{O}\) — or, direct from the oxide, \(\text{P}_4\text{O}_{10} + 12\text{NaOH} \rightarrow 4\text{Na}_3\text{PO}_4 + 6\text{H}_2\text{O}\). The word 'excess' is what licenses the fully deprotonated salt; with limited alkali you would stop at NaH\(_2\)PO\(_4\) or Na\(_2\)HPO\(_4\). Identification, balanced equations, pH logic — that is the full anatomy of an AQA 3.2.4 structured question.

VocabularyKey terms the mark scheme pays for

Giant ionic lattice

The structure of Na₂O, MgO and Al₂O₃: oppositely charged ions held by electrostatic attraction in all directions, giving very high melting points.

Macromolecular (giant covalent)

The structure of SiO₂: a continuous 3-D network of covalent bonds, so melting requires breaking strong covalent bonds — high melting point, insoluble in water.

Simple molecular oxide

P₄O₁₀, SO₂ and SO₃: strong covalent bonds within molecules but weak intermolecular forces between them, so low melting points despite strong bonds.

Basic oxide

A metal oxide whose oxide ions accept protons — Na₂O and MgO react with water to give alkaline solutions and neutralise acids to give a salt plus water.

Acidic oxide

A non-metal oxide that reacts with water to form an acid (P₄O₁₀ → H₃PO₄) or reacts directly with bases — SO₂, SO₃ and, with hot concentrated alkali, SiO₂.

Amphoteric oxide

An oxide that reacts with both acids and bases — Al₂O₃ gives aluminium sulfate with sulfuric acid and sodium aluminate with sodium hydroxide.

Sparingly soluble

Dissolving only slightly — Mg(OH)₂ in water. The low solubility, not just slow reaction, is why magnesium's solution reaches only pH 9–10 against sodium's 13–14.

Covalent character

Distortion of an anion's electron cloud by a small, highly charged cation such as Al³⁺, shifting ionic bonding towards electron sharing and lowering Al₂O₃'s melting point below MgO's.

TrapsMisconceptions that cost marks

“Magnesium doesn't react with water.”

Actually: It reacts twice over — just not dramatically in the cold. With cold water it forms Mg(OH)₂ and hydrogen extremely slowly; with steam it burns rapidly to MgO and hydrogen. Two different products, two different equations, and examiners test the difference.

“Aluminium oxide and silicon dioxide are neutral because the water stays at pH 7.”

Actually: Insoluble is not neutral. Al₂O₃ is amphoteric — it reacts with both sulfuric acid and sodium hydroxide — and SiO₂ is an acidic oxide that dissolves in hot concentrated NaOH to give sodium silicate. pH of the water only tells you about solubility.

“MgO has a higher melting point than Al₂O₃ because 2+ beats 3+... so the trend must be wrong.”

Actually: The trend isn't charge alone. Al³⁺ is so small and highly charged that it polarises the oxide ion, giving Al₂O₃ partial covalent character that weakens the purely ionic attraction — so its melting point (≈2,070 °C) falls below MgO's (≈2,850 °C).

ExamWhat examiners want

AQA marks 3.2.4 answers as trios: observation, balanced equation, pH. For sodium in water, 'fizzes, melts into a ball' + the equation + 'pH 13–14' is three marks; any one alone is not. Learn the approximate pH of every oxide-in-water outcome — 13–14 (NaOH), 9–10 (sparingly soluble Mg(OH)₂), 7 (insoluble Al₂O₃ and SiO₂), 0–1 (H₃PO₄, H₂SO₄), 2–3 (H₂SO₃) — because pH values are AO1 recall the paper asks for directly.

The six-mark extended response here is almost always 'explain the trend in melting points of the Period 3 oxides' or 'describe the acid-base chemistry across the period'. Structure the answer by the three families — ionic, macromolecular, molecular — and for each give the structure, the force being overcome, and a supporting value or equation. Examiner reports repeatedly credit answers that say what is broken on melting (attraction between ions; covalent bonds; intermolecular forces) and penalise answers that say 'ionic bonds are strong' without naming the electrostatic attraction.

In equation questions, the classic dropped marks are state symbols when steam is involved — write H₂O(g) for magnesium in steam — the 'sparingly soluble' wording for Mg(OH)₂, and the amphoteric pair for Al₂O₃: one equation with an acid AND one with a base. If the question says 'excess' sodium hydroxide, take acids like H₃PO₄ all the way to the fully substituted salt.