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Period 3 oxides: structure and acid-base characterAQA A-Level Chemistry: Revision notes

Section 1

Structure and melting point of the oxides

The highest oxides of the Period 3 elements Na to S change structure across the period:

  • Na₂O, MgO are giant ionic lattices. Strong electrostatic attraction between oppositely charged ions gives high melting points. MgO is higher than Na₂O because Mg²⁺ has a greater charge and a smaller radius.
  • Al₂O₃ is giant ionic with some covalent character and has a very high melting point.
  • SiO₂ is giant covalent. Many strong covalent bonds must be broken, so the melting point is very high.
  • P₄O₁₀ and SO₃ are simple molecular. Only weak intermolecular forces are overcome on melting, so the melting points are low. P₄O₁₀ melts higher than SO₃ because its molecules are larger with stronger London forces.
Key termsgiant ionic latticegiant covalent
Common mistake

For P₄O₁₀ and SO₃ do not write that covalent bonds are broken on melting. The molecules stay intact.

Section 2

Reactions of the oxides with water

  • Na₂O reacts to give a strongly alkaline solution: Na₂O + H₂O → 2NaOH (pH about 14).
  • MgO reacts slightly: MgO + H₂O → Mg(OH)₂ (pH about 9 to 10), as Mg(OH)₂ is only sparingly soluble.
  • Al₂O₃ and SiO₂ are insoluble in water, so the pH stays about 7.
  • P₄O₁₀ gives an acidic solution: P₄O₁₀ + 6H₂O → 4H₃PO₄ (pH about 0 to 2).
  • SO₂ gives a weakly acidic solution: SO₂ + H₂O → H₂SO₃ (pH about 3).
  • SO₃ gives a strongly acidic solution: SO₃ + H₂O → H₂SO₄ (pH about 0).

Ionic oxides contain O²⁻, which accepts H⁺ from water: O²⁻ + H₂O → 2OH⁻.

Key termsbasic oxideacidic oxide

Section 3

Structures of the acids and anions

  • H₃PO₄ (phosphoric(V) acid): P bonded to four oxygen atoms: one P=O and three P–OH. The anion is PO₄³⁻ (phosphate(V)).
  • H₂SO₃ (sulfuric(IV) acid): S bonded to three oxygen atoms: one S=O and two S–OH, with a lone pair on S. The anion is SO₃²⁻ (sulfate(IV)).
  • H₂SO₄ (sulfuric(VI) acid): S bonded to four oxygen atoms: two S=O and two S–OH. The anion is SO₄²⁻ (sulfate(VI)).

Loss of H⁺ from each O–H gives the anion. The phosphate(V) and sulfate(VI) ions are tetrahedral.

Key termssulfate(IV)

Section 4

Acid-base character and bonding

Across the period the oxides change from basic (Na₂O, MgO) to amphoteric (Al₂O₃) to acidic (SiO₂, P₄O₁₀, SO₂, SO₃).

This follows the change from ionic bonding (metal oxides, containing O²⁻ that accepts protons) to covalent bonding (non-metal oxides that form acids with water or react with bases).

An amphoteric oxide reacts with both acids and bases. Al₂O₃ is insoluble in water but dissolves in both.

Key termsamphoteric

Section 5

Equations with acids and bases

  • Basic oxides with acid: MgO + 2HCl → MgCl₂ + H₂O; Na₂O + 2HCl → 2NaCl + H₂O
  • Amphoteric: Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O; Al₂O₃ + 2NaOH + 3H₂O → 2NaAl(OH)₄
  • Acidic oxides with base: SiO₂ + 2NaOH → Na₂SiO₃ + H₂O; P₄O₁₀ + 12NaOH → 4Na₃PO₄ + 6H₂O; SO₂ + 2NaOH → Na₂SO₃ + H₂O; SO₃ + 2NaOH → Na₂SO₄ + H₂O
Exam tip

Balance equations by atoms and charge. P₄O₁₀ + 12NaOH uses 12 NaOH because the P₄ unit makes four phosphate ions, each needing three Na⁺.

Must Know

  • Na₂O, MgO giant ionic; Al₂O₃ ionic; SiO₂ giant covalent; P₄O₁₀ and SO₃ simple molecular
  • Melting points are high up to SiO₂, then much lower for P₄O₁₀ and SO₃
  • Na₂O and MgO basic; Al₂O₃ amphoteric; SiO₂, P₄O₁₀, SO₂, SO₃ acidic
  • P₄O₁₀ gives H₃PO₄, SO₂ gives H₂SO₃, SO₃ gives H₂SO₄

That's the notes covered.

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Exam questions on Period 3 oxides: structure and acid-base character

  1. A student is given samples of four Period 3 oxides, sodium oxide, magnesium oxide, silicon dioxide and phosphorus(V) oxide, and is asked to compare their melting points.
    Explain why the melting point of magnesium oxide is higher than that of sodium oxide.2 marks
  2. A student adds small amounts of sodium oxide, aluminium oxide and phosphorus(V) oxide to separate beakers of water and tests the pH of each mixture with universal indicator.
    Write an equation for the reaction of sodium oxide with water and explain why the resulting solution is alkaline.2 marks
  3. A technician is preparing a safety sheet for the oxides of Period 3. She needs equations for the acids formed when phosphorus(V) oxide, sulfur dioxide and sulfur trioxide dissolve in water, and a description of the structures of some of the acids and anions involved.
    Write equations for the reactions of phosphorus(V) oxide, sulfur dioxide and sulfur trioxide with water.3 marks
See the full worksheet

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).