AQA A-Level Chemistry: Properties of Period 3 elements and their oxides (7405) – Revision Notes

These notes condense section 3.2.4 Properties of Period 3 elements and their oxides of the AQA AS and A-level Chemistry specification (7404/7405), version 1.2, July 2026, for AS and A-level exams June 2016 onwards. The whole section is A-level only, so it is not in the AS course. Inorganic chemistry is on A-level Paper 1, and A-level Paper 3 can test any content.

For full explanations and worked examples, read the Period 3 oxides study guide. To practise exam-style questions, use the Period 3 oxides practice questions. The course hub is AQA A-Level Chemistry, the printable checklist lists every specification point, and the free 10-minute diagnostics show where your gaps are. The physical trends across Period 3 (atomic radius, ionisation energy, melting points of the elements) are in the periodicity revision notes.

Key terms

• Basic oxide: reacts with acids to form a salt and water (Na₂O, MgO).
• Acidic oxide: reacts with bases to form a salt and water (SiO₂, P₄O₁₀, SO₂, SO₃).
• Amphoteric oxide: reacts with both acids and bases (Al₂O₃).
• Highest oxide: the oxide with the element in its maximum oxidation state (Na₂O, MgO, Al₂O₃, SiO₂, P₄O₁₀, SO₃).

Sodium and magnesium with water

Reaction	Equation	What you see / pH
Na + cold water	2Na + 2H₂O → 2NaOH + H₂	vigorous; floats, melts, fizzes; pH 13 to 14
Mg + cold water	Mg + 2H₂O → Mg(OH)₂ + H₂	very slow, few bubbles; pH 9 to 10
Mg + steam	Mg + H₂O → MgO + H₂	burns with bright white flame; white solid

Why Na is faster: it loses only one electron per atom. Mg must lose two, and the total energy needed is much larger.

Elements with oxygen

Element	Equation	Flame / product
Na	4Na + O₂ → 2Na₂O	yellow-orange; white solid
Mg	2Mg + O₂ → 2MgO	bright white; white solid
Al	4Al + 3O₂ → 2Al₂O₃	powder burns brightly; white solid
Si	Si + O₂ → SiO₂	strong heating needed; white solid
P	P₄ + 5O₂ → P₄O₁₀	bright flame; white fumes
S	S + O₂ → SO₂	blue flame; colourless choking gas
SO₂ → SO₃	2SO₂ + O₂ ⇌ 2SO₃	needs V₂O₅ catalyst and heat

Maximum oxidation state: Na +1, Mg +2, Al +3, Si +4, P +5, S +6. It equals the number of outer electrons. Bulk aluminium is protected by a thin Al₂O₃ layer.

Melting points of the highest oxides

Order: MgO > Al₂O₃ > SiO₂ > Na₂O > P₄O₁₀ > SO₃ (SO₂ is lower still).

Oxide	Structure	Why that melting point
Na₂O	giant ionic	strong attraction between Na⁺ and O²⁻
MgO	giant ionic	Mg²⁺ has higher charge and is smaller than Na⁺, so stronger attraction than in Na₂O
Al₂O₃	giant ionic, some covalent character	Al³⁺ polarises O²⁻; high, but below MgO
SiO₂	macromolecular	many strong covalent bonds to break
P₄O₁₀	simple molecular	van der Waals only; bigger molecule (more electrons) than SO₃
SO₃	simple molecular	weak van der Waals; low

Method in steps: explaining a melting point

1. Name the structure (giant ionic, macromolecular or simple molecular).
2. Name what is overcome on melting (ionic attraction, covalent bonds, or intermolecular forces).
3. Say how strong it is and so how much energy is needed.
4. For a comparison, give the reason one is stronger (ion charge and size; number of electrons).

Oxides with water

Oxide	Equation	pH	Bonding
Na₂O	Na₂O + H₂O → 2NaOH	13 to 14	ionic
MgO	MgO + H₂O → Mg(OH)₂	9 to 10	ionic
Al₂O₃	insoluble	7	ionic with covalent character
SiO₂	insoluble	7	macromolecular covalent
P₄O₁₀	P₄O₁₀ + 6H₂O → 4H₃PO₄	0 to 1	simple molecular covalent
SO₂	SO₂ + H₂O → H₂SO₃	2 to 3	simple molecular covalent
SO₃	SO₃ + H₂O → H₂SO₄	0 to 1	simple molecular covalent

• Ionic oxides: O²⁻ + H₂O → 2OH⁻, so alkaline. MgO gives a lower pH because Mg(OH)₂ is only slightly soluble.
• Giant structures Al₂O₃ and SiO₂: too strongly bonded to dissolve; pH stays 7.
• Simple covalent oxides: form acids that release H⁺. H₂SO₃ is weak (H₂SO₃ ⇌ H⁺ + HSO₃⁻), so pH is higher than for H₂SO₄.

Acid and anion structures

Acid	Fully deprotonated anion	Other anions	Shape at P or S
H₃PO₄: P=O plus three P–OH	PO₄³⁻: P=O plus three P–O⁻	H₂PO₄⁻, HPO₄²⁻	tetrahedral
H₂SO₃: S=O plus two S–OH, lone pair on S	SO₃²⁻: S=O plus two S–O⁻, lone pair	HSO₃⁻	pyramidal
H₂SO₄: two S=O plus two S–OH	SO₄²⁻: two S=O plus two S–O⁻	HSO₄⁻	tetrahedral

Quick sketch of H₂SO₃ to remember the lone pair:

         O
         ‖
  H–O – S – O–H      with one lone pair on S

Acid and base equations

Oxide	With acid	With base
Na₂O	Na₂O + H₂SO₄ → Na₂SO₄ + H₂O	no reaction
MgO	MgO + 2HNO₃ → Mg(NO₃)₂ + H₂O	no reaction
Al₂O₃	Al₂O₃ + 6H⁺ → 2Al³⁺ + 3H₂O	Al₂O₃ + 2OH⁻ + 3H₂O → 2Al(OH)₄⁻
SiO₂	no reaction	SiO₂ + 2OH⁻ → SiO₃²⁻ + H₂O (hot, concentrated)
P₄O₁₀	no reaction	P₄O₁₀ + 6Na₂O → 4Na₃PO₄
SO₂	no reaction	SO₂ + 2OH⁻ → SO₃²⁻ + H₂O; with less base, SO₂ + NaOH → NaHSO₃
SO₃	no reaction	SO₃ + CaO → CaSO₄

Method in steps: balancing an oxide + base equation

1. Write the salt: the anion keeps the oxidation state of P or S (S +4 → SO₃²⁻; S +6 → SO₄²⁻; P +5 → PO₄³⁻).
2. Balance the central atom first (P₄O₁₀ gives 4 PO₄³⁻).
3. Balance the metal, then hydrogen with water.
4. Check that charges and oxygen atoms balance.

Practical link

There is no required practical in section 3.2.4, but the specification suggests burning elements in oxygen and testing the pH of the oxides (AT a, c and k; PS 2.2).

• Burn the element on a combustion spoon in a gas jar of oxygen, add distilled water and shake.
• Measure pH with universal indicator and a pH chart, or with a pH meter or probe.
• Burn phosphorus and sulfur in a fume cupboard: their oxides are toxic and irritant.
• Present results in a table: element, observation, oxide, pH, and whether the oxide is acidic or basic.
• Expected pattern: alkaline for Na and Mg, neutral for Al and Si (insoluble oxides), acidic for P and S.

Must-know distinctions

• Mg + cold water gives Mg(OH)₂; Mg + steam gives MgO.
• SO₂ → H₂SO₃ (S +4, weak acid); SO₃ → H₂SO₄ (S +6).
• Insoluble (Al₂O₃, SiO₂: pH 7) is not the same as neutral oxide: Al₂O₃ is amphoteric and SiO₂ is acidic.
• Al₂O₃ and SiO₂ melting both need lots of energy, but for different reasons: ionic attraction against covalent bonds.
• Simple molecular melting overcomes forces between molecules, never the bonds inside them.

Quick self-test

1. Write an equation for the reaction of magnesium with steam.
2. Give the approximate pH of the solution formed when Na₂O is added to water.
3. Name the oxide of Period 3 with the highest melting point.
4. State the type of structure in SiO₂.
5. Write an equation for the reaction of SO₃ with water.
6. Why does aluminium oxide not change the pH of water?
7. State the shape around sulfur in the SO₃²⁻ ion.
8. Write an ionic equation for Al₂O₃ reacting with hydroxide ions.
9. How many moles of hydrogen form when 0.46 g of sodium reacts with water? (Ar Na = 23.0)
10. How many moles of NaOH react with 0.010 mol of Al₂O₃ to form NaAl(OH)₄?
11. How many moles of H₃PO₄ form from 0.25 mol of P₄O₁₀ with excess water?
12. Why does MgO give a lower pH than Na₂O in water?

Answers

1. Mg + H₂O → MgO + H₂
2. About 13 to 14.
3. MgO.
4. Macromolecular (giant covalent).
5. SO₃ + H₂O → H₂SO₄
6. It is insoluble, because of its strong bonding in a giant lattice.
7. Pyramidal (three bonding regions and one lone pair).
8. Al₂O₃ + 2OH⁻ + 3H₂O → 2Al(OH)₄⁻
9. 0.46 / 23.0 = 0.020 mol Na; 2Na → H₂, so 0.010 mol H₂.
10. Al₂O₃ : NaOH = 1 : 2, so 0.020 mol.
11. P₄O₁₀ : H₃PO₄ = 1 : 4, so 1.0 mol.
12. Mg(OH)₂ is only slightly soluble, so fewer OH⁻ ions are in solution.

Where marks are usually lost

• Using “MgO” as the product of magnesium and cold water.
• Writing H₂SO₄ for the product of SO₂ and water.
• Describing Al₂O₃ and SiO₂ as “neutral oxides” because the pH is 7.
• Explaining the low melting point of P₄O₁₀ by “weak covalent bonds”.
• Comparing MgO and Na₂O without mentioning both ion charge and ion size.
• Leaving out water in Al₂O₃ + 2NaOH + 3H₂O → 2NaAl(OH)₄.
• Drawing H₂SO₃ or SO₃²⁻ without the lone pair on sulfur.
• Giving the wrong anion charge: sulfate(IV) is SO₃²⁻ and phosphate is PO₄³⁻.
• Forgetting that SiO₂ needs hot, concentrated NaOH and does not react with water.

Official syllabus

AQA, AS and A-level Chemistry specification (7404/7405), version 1.2, July 2026, for AS and A-level exams June 2016 onwards: section 3.2.4 Properties of Period 3 elements and their oxides (A-level only).