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2.4.1Inorganic Chemistry

Properties of Period 3 Elements and Their Oxides and Chlorides — Oxford AQA International A Level Chemistry Revision Notes & Topical Questions

5 min read 5 parts Lesson 15 of 27

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Part 1 of 5Sodium and Magnesium with Water

2.4.1 Properties of Period 3 Elements and Their Oxides and Chlorides
Sodium
  • 2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g);
    • a vigorous reaction in which the metal melts, fizzes and moves across the surface.
Magnesium and cold water
  • The reaction is very slow and only a few bubbles form, giving a weakly alkaline solution.
Magnesium and steam
  • Mg(s) + H₂O(g) → MgO(s) + H₂(g); the metal glows brightly and forms white magnesium oxide.
Explanation
  • Sodium loses one electron from a larger atom, whereas magnesium needs more energy to lose two.

Part 2 of 5Reactions with Oxygen

Reactions with Oxygen
Sodium
  • Burns with a yellow flame to form the white solid Na₂O: 4Na + O₂ → 2Na₂O.
Magnesium
  • Burns with a brilliant white flame2Mg + O₂ → 2MgO.
Phosphorus
  • Burns to form white fumes of P₄O₁₀: P₄ + 5O₂ → P₄O₁₀.
Sulfur
  • Burns with a blue flame to form the colourless gas SO₂: S + O₂ → SO₂.

Part 3 of 5Structure of Oxides

Na₂O, MgO
  • Giant ionic lattices with high melting points that conduct when molten;
    • MgO has the stronger lattice because of its 2+ and 2− ions.
Al₂O₃
  • An ionic oxide with some covalent character, which gives a very high melting point.
SiO₂
  • A giant covalent network with strong bonds throughout and a very high melting point.
P₄O₁₀, SO₂, SO₃
  • Simple molecular oxides with weak London forces between molecules, so melting points are low.
OxideBondingConducts when molten?
Na₂O, MgOIonicYes
Al₂O₃Ionic/covalentYes
SiO₂Covalent networkNo
P₄O₁₀, SO₂MolecularNo

Part 4 of 5Structure of Chlorides

Structure of Chlorides
Ionic chlorides
  • NaCl and MgCl₂ are ionic solids with high melting points that conduct when molten or in aqueous solution.
Aluminium chloride
  • AlCl₃ has covalent character and exists as dimers, Al₂Cl₆, in the vapour, so it is more volatile than expected.
Molecular chlorides
  • SiCl₄ and PCl₅ are covalent molecular substances, and they are liquids or low-melting solids that do not conduct.
Trend
  • Bonding changes from ionic to covalent across the period as the cation becomes more polarising.

Part 5 of 5Exam Checklist & Tips

2.4.1 – Key Points for the Exam

  • Period 3 The row of elements from sodium to argon.
  • Giant ionic lattice A regular 3D arrangement of oppositely charged ions held by strong electrostatic attraction.
  • Giant covalent A network of atoms bonded by covalent bonds throughout.
  • Simple molecular A substance made of small molecules held by weak intermolecular forces.

Common Mistakes

  • Saying Na reacts with water to give an oxide rather than hydroxide.
  • Stating that silicon dioxide is a simple molecular compound.
  • Claiming that molecular oxides conduct electricity.
  • Forgetting that aluminium chloride forms a dimer.

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