Periodicity of Period 3 Elements

Reactions with Water

• Sodium (Na):
  • Reacts with cold water.
  • Produces sodium hydroxide and hydrogen gas.
  • Equation: 2 Na (s) + 2 H₂O (l) → 2 NaOH (aq) + H₂ (g)
• Magnesium (Mg):
  • Reacts very slowly with cold water to form magnesium hydroxide.
  • Reacts more readily with steam to form magnesium oxide.
  • Equation: Mg (s) + H₂O (g) → MgO (s) + H₂ (g)

Reactions with Oxygen

• All elements react with oxygen to form oxides.
• Sodium (Na):
  • Burns with a yellow flame.
  • Produces a white solid.
  • Equation: 4 Na (s) + O₂ (g) → 2 Na₂O (s)
• Magnesium (Mg), Aluminium (Al), Silicon (Si), Phosphorus (P):
  • Burn with a white flame to produce white solid smoke.
  • Equations:
    • 2 Mg (s) + O₂ (g) → 2 MgO (s)
    • 4 Al (s) + 3 O₂ (g) → 2 Al₂O₃ (s)
    • Si (s) + O₂ (g) → SiO₂ (s)
    • 4 P (s) + 5 O₂ (g) → P₄O₁₀ (s)
• Sulfur (S):
  • Burns with a blue flame.
  • Produces an acidic choking gas.
  • Equation: S (s) + O₂ (g) → SO₂ (g)

Properties of Period 3 Oxides

• Ionic Oxides:
  • Na₂O, MgO, Al₂O₃ are ionic.
  • High melting points due to ionic giant lattice structures.
  • Melting points increase from Na to Al due to increasing cation charge.
  • Al₂O₃ shows some covalent character.
• Macromolecular Oxides:
  • SiO₂ is macromolecular with very strong covalent bonds.
  • Very high melting and boiling points.
• Simple Molecular Oxides:
  • P₄O₁₀, SO₂ have weak intermolecular forces.
  • Lower melting points.
  • Covalent due to small electronegativity difference.

Acid-Base Reactions

• Basic oxides react with acids to make salts.
  • Equations:
    • Na₂O (s) + 2 HCl (aq) → 2 NaCl (aq) + H₂O (l)
    • MgO (s) + 2 HCl (aq) → MgCl₂ (aq) + H₂O (l)
• Amphoteric Oxides:
  • Al₂O₃ can act as both an acid and a base.
  • Acting as a base:
    • Al₂O₃ + 6 HCl → 2 AlCl₃ + 3 H₂O
  • Acting as an acid:
    • Al₂O₃ + 2 NaOH + 3 H₂O → 2 NaAl(OH)₄

Reactions with Water

• Ionic metal oxides tend to form hydroxides which are alkaline.
  • Na₂O + H₂O → 2 Na⁺ + 2 OH⁻ (pH 13)
  • MgO + H₂O → Mg(OH)₂ (pH 9)
  • Al₂O₃ and SiO₂ do not dissolve in water.
• Non-metal covalent oxides react with water to form acids.
  • P₄O₁₀ (s) + 6 H₂O (l) → 4 H₃PO₄ (aq) (pH 0)
  • SO₂ (g) + H₂O (l) → H₂SO₃ (aq) (pH 3)
  • SO₃ (g) + H₂O (l) → H₂SO₄ (aq) (pH 0)

Other Key Points

• Ionic oxides act as Brønsted-Lowry bases by accepting protons.
• Aluminium metal is protected by a thin layer of aluminium oxide.
• Sodium is stored under oil, and phosphorus under water to prevent reactions with air.
• SiO₂ reacts with concentrated NaOH but not with water or weak alkali solutions.
• MgO is preferred for treating acid in rivers and the stomach due to its weak alkalinity.