Periodicity of chemical properties of the elements in Period 3

AS Level Chemistry · The Periodic Table: chemical periodicity. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

Across Period 3, from sodium to sulfur, the elements change from metals to non-metals. Their oxides and chlorides change with them: ionic solids on the left, covalent compounds on the right. This happens because the electronegativity of the element gets closer to that of oxygen or chlorine.

Ionic oxides are basic and covalent oxides are acidic. Aluminium oxide is in between: it is amphoteric, so it reacts with both acids and alkalis. Ionic chlorides simply dissolve in water. Covalent chlorides are hydrolysed by water and release HCl, so the solution is strongly acidic.

The highest oxidation number of each element equals its number of outer-shell electrons.

Rules to remember

Common mistake

Students often say that Al2O3 and SiO2 change the pH of water. They are insoluble and do not react with water, so the pH stays at 7, even though Al2O3 is amphoteric and SiO2 is acidic.

Worked example

An oxide of a Period 3 element is a white solid that reacts violently with water to give a solution of pH about 1. The element has oxidation number +5 in the oxide. Identify the oxide and write the equation for its reaction with water.

  1. A strongly acidic solution means a covalent oxide of a non-metal, so it is an oxide of phosphorus or sulfur.
  2. Oxidation number +5 means five outer-shell electrons, which is phosphorus (sulfur gives +4 or +6).
  3. Check in P4O10: 4x + 10 × (−2) = 0, so x = +5.

Answer: The oxide is P4O10: P4O10 + 6H2O → 4H3PO4.

Practice questions

Try each one, then open the answer.

1. Aluminium reacts with chlorine to form Al2Cl6. How many moles of chlorine molecules react completely with 0.10 mol of aluminium?

  1. A
    0.05
  2. B
    0.10
  3. C
    0.15
  4. D
    0.30
Show answer

Answer: C. 2Al + 3Cl2 → Al2Cl6, so moles of Cl2 = 0.10 × 3/2 = 0.15 mol. 0.30 mol counts chlorine atoms, not molecules.

2. Which equation shows aluminium oxide behaving as an acid?

  1. A
    Al2O3 + 6HCl → 2AlCl3 + 3H2O
  2. B
    Al2O3 + 2NaOH + 3H2O → 2NaAl(OH)4
  3. C
    Al2O3 + 3H2O → 2Al(OH)3
  4. D
    2Al + 3Cl2 → 2AlCl3
Show answer

Answer: B. Aluminium oxide is amphoteric: it reacts with acids as a base and with alkalis as an acid, forming the aluminate ion. It does not react with water at all.

3. Which Period 3 hydroxide reacts with both dilute hydrochloric acid and aqueous sodium hydroxide?

  1. A
    NaOH
  2. B
    Mg(OH)2
  3. C
    both Mg(OH)2 and Al(OH)3
  4. D
    Al(OH)3
Show answer

Answer: D. Al(OH)3 is amphoteric: with HCl it gives AlCl3 and water, and with NaOH it gives NaAl(OH)4. Mg(OH)2 is basic only, so it reacts with acids but not with alkalis.

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