Topic 2 of 3
Period 3 compounds
Compare every required oxide, hydroxide and chloride.
A-Level 8873, revised syllabus (2026-2027)
Move from basic oxides to acidic oxides
Track oxidation number, bonding and the reactions with water, acid and alkali.
In these highest oxides, oxygen has oxidation number -2. The element's highest oxidation number rises across the period: Na +1, Mg +2, Al +3, Si +4, P +5 and S +6. More outer electrons can participate in bonding across this sequence. With oxygen fixed, the electronegativity difference generally decreases, and bonding changes from predominantly ionic to covalent.
| Oxide; element oxidation number | Bonding/structure | With water |
|---|---|---|
| Na2O; +1 | Giant ionic | Na2O(s) + H2O(l) → 2NaOH(aq) |
| MgO; +2 | Giant ionic | MgO(s) + H2O(l) → Mg(OH)2(s); slow reaction, sparingly soluble hydroxide makes the water weakly alkaline |
| Al2O3; +3 | Predominantly ionic giant structure | No appreciable reaction with water |
| SiO2; +4 | Giant covalent | No reaction with water |
| P4O10; +5 | Covalent, molecular | P4O10(s) + 6H2O(l) → 4H3PO4(aq) |
| SO3; +6 | Covalent | SO3(g) + H2O(l) → H2SO4(aq); rapid and strongly exothermic when gaseous SO3 contacts water |
Na2O and MgO are basic: their oxide ions accept protons. For example, MgO(s) + 2H+(aq) → Mg2+(aq) + H2O(l). Their corresponding hydroxides neutralise acids: NaOH(aq) + H+(aq) → Na+(aq) + H2O(l), and Mg(OH)2(s) + 2H+(aq) → Mg2+(aq) + 2H2O(l). Magnesium hydroxide is only sparingly soluble, so an excess solid does not make an arbitrarily concentrated solution.
Al2O3 and Al(OH)3 are amphoteric: each reacts with acid and with strong alkali. The specified alkali is sodium hydroxide; heat the oxide with concentrated aqueous NaOH, whereas freshly precipitated Al(OH)3 dissolves in excess aqueous NaOH. With acid, Al2O3 + 6H+ → 2Al3+ + 3H2O; Al(OH)3 + 3H+ → Al3+ + 3H2O. In aqueous hydroxide, soluble aluminate species form.
| Starting solid | Balanced ionic equation |
|---|---|
| Al(OH)3 | Al(OH)3(s) + OH-(aq) → [Al(OH)4]-(aq) |
| Al2O3 | Al2O3(s) + 2OH-(aq) + 3H2O(l) → 2[Al(OH)4]-(aq) |
SiO2, P4O10 and SO3 are acidic oxides and react with alkali. Insoluble SiO2 does not need to react with water to be acidic: SiO2 + 2NaOH → Na2SiO3 + H2O with hot concentrated NaOH (or fused NaOH). With sufficient NaOH, P4O10 + 12NaOH → 4Na3PO4 + 6H2O; SO3 + 2NaOH → Na2SO4 + H2O.
Worked example
Predict from a pair of observations
A white oxide does not react with water. It dissolves in acid and also in aqueous NaOH. Which specified Period 3 oxide fits?
- No reaction with water alone leaves more than one candidate.
- Reaction with both acid and alkali establishes amphoteric behaviour.
- Of the named Period 3 oxides, Al2O3 fits.
Al2O3. The two chemical tests are more diagnostic than colour or insolubility alone.
Dissolving and hydrolysing are different processes
The pH after adding water reveals more than whether the solid disappears.
| Chloride; highest oxidation number | Bonding | Behaviour with water |
|---|---|---|
| NaCl; Na +1 | Ionic | Dissolves to Na+ and Cl-; approximately neutral solution. |
| MgCl2; Mg +2 | Ionic | Dissolves; slight hydrolysis of hydrated Mg2+ can make the solution mildly acidic. |
| AlCl3; Al +3 | Covalent when anhydrous | Dissolves and forms hydrated Al3+; marked hydrolysis produces an acidic solution. |
| SiCl4; Si +4 | Covalent molecular | Rapid hydrolysis: SiCl4(l) + 2H2O(l) → SiO2(s) + 4HCl(aq). |
| PCl5; P +5 | Covalent in the molecular representation used for reactions | With excess water: PCl5(s) + 4H2O(l) → H3PO4(aq) + 5HCl(aq). |
Across the series the highest element oxidation number rises with the available outer electrons. Increasing electronegativity of the Period 3 element generally reduces the difference from chlorine and favours covalent bonding. AlCl3 is the stated exception to the simple electronegativity classification; the small, highly charged aluminium centre strongly polarises chloride electron density. Its molecular dimer Al2Cl6 is discussed in Bonding.
For NaCl, separating ions into water is principally dissolution. A hydrated Mg2+ or Al3+ ion attracts electron density from its water ligands, weakening O-H bonds and allowing proton release. The effect is stronger for Al3+, with greater charge density. It is the hydrated cation, not Cl- simply splitting into HCl, that explains this acidity.
For example, [Al(H2O)6]3+ + H2O ⇌ [Al(H2O)5(OH)]2+ + H3O+. The equation makes proton release and charge conservation explicit. By contrast, SiCl4 and PCl5 undergo extensive hydrolysis of their covalent chlorides.