Periodicity of Chemical Properties of the Elements in Period 3

2026 Syllabus Objectives

By the end of this topic, you should be able to:

  1. Describe and write equations for the reactions of Period 3 elements with oxygen, chlorine, and water
  2. State and explain the oxidation numbers in Period 3 oxides and chlorides
  3. Describe and write equations for reactions of Period 3 oxides with water, including pH values
  4. Describe and explain the acid/base behaviour of Period 3 oxides and hydroxides
  5. Describe and explain the reactions of Period 3 chlorides with water, including pH values
  6. Explain trends in terms of bonding and electronegativity
  7. Suggest types of bonding from physical and chemical properties

1. Reactions of Period 3 Elements

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Reactions with Oxygen

Period 3 elements react with oxygen to form oxides. The reactivity and conditions vary across the period.

Sodium (Na):

  • Equation: 4Na(s) + O₂(g) → 2Na₂O(s)
  • Conditions: Heat
  • Observations: Vigorous reaction with a bright yellow flame, forming a white solid

Magnesium (Mg):

  • Equation: 2Mg(s) + O₂(g) → 2MgO(s)
  • Conditions: Heat
  • Observations: Vigorous reaction with a bright white flame, forming a white solid

Aluminium (Al):

  • Equation: 4Al(s) + 3O₂(g) → 2Al₂O₃(s)
  • Conditions: Powdered aluminium
  • Observations: Fast reaction with a bright white flame, forming a white powder

Phosphorus (P):

  • Equation: P₄(s) + 5O₂(g) → P₄O₁₀(s)
  • Conditions: Heat with excess oxygen
  • Observations: Vigorous reaction with a yellow or white flame, forming white clouds
  • Important Note: Always write phosphorus as P₄ (not just P) because phosphorus exists as P₄ molecules. Also use the molecular formula P₄O₁₀ (not the empirical formula P₂O₅).

Sulfur (S):

  • Equation: S(s) + O₂(g) → SO₂(g)
  • Conditions: Powdered sulfur with heat
  • Observations: Gentle reaction with a blue flame, forming toxic fumes

Reactions with Chlorine

Period 3 elements also react with chlorine gas to form chlorides.

Sodium (Na):

  • Equation: 2Na(s) + Cl₂(g) → 2NaCl(s)
  • Conditions: Heat
  • Observation: Vigorous reaction

Magnesium (Mg):

  • Equation: Mg(s) + Cl₂(g) → MgCl₂(s)
  • Conditions: Heat
  • Observation: Vigorous reaction

Aluminium (Al):

  • Equation: 2Al(s) + 3Cl₂(g) → 2AlCl₃(s)
  • Conditions: Heat
  • Observation: Vigorous reaction
  • Note: Aluminium chloride often exists as Al₂Cl₆ dimers (pairs of molecules joined together), but you can write it as AlCl₃ in equations.

Silicon (Si):

  • Equation: Si(s) + 2Cl₂(g) → SiCl₄(l)
  • Conditions: Heat
  • Observation: Slow reaction

Phosphorus (P):

  • Equation: P₄(s) + 10Cl₂(g) → 4PCl₅(s)
  • Conditions: Heat with excess chlorine
  • Observation: Slow reaction

Reactions with Water

Only sodium and magnesium react with water under normal laboratory conditions.

Sodium (Na):

  • Equation: 2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)
  • Observations:
    • The sodium melts into a ball
    • It moves quickly across the water surface
    • Hydrogen gas is released (fizzing)
    • The reaction is vigorous
    • The solution becomes strongly alkaline with pH 14
    • This is because sodium hydroxide (NaOH) is formed

Magnesium (Mg):

  • With cold water: Mg(s) + 2H₂O(l) → Mg(OH)₂(aq) + H₂(g)

    • This reaction is extremely slow
    • The solution is weakly alkaline with pH 11
    • Magnesium hydroxide is only slightly soluble in water, so the pH is not as high as with sodium

  • With steam (hot water vapour): Mg(s) + H₂O(g) → MgO(s) + H₂(g)

    • When heated, magnesium reacts vigorously with steam
    • This produces magnesium oxide (not the hydroxide) and hydrogen gas

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