Lesson 2 of 3

Period 3 compounds

Compare every required oxide, hydroxide and chloride.

A-Level 8873, revised syllabus (2026-2027)

I can connect Period 3 compounds to their behaviour.Syllabus
Syllabus 8873, 4(e). Connect Period 3 compounds to their behaviour.

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.

Every specified oxide
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.

Amphoteric reactions in aqueous NaOH
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 examplePredict 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?

Try it first. Then show the worked stepsHide the worked steps

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.

Answer

Al₂O₃. The two chemical tests are more diagnostic than colour or insolubility alone.

Check your understanding

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

Aluminium oxide, Al₂O₃
Sulfur trioxide, SO₃
Magnesium oxide, MgO

Show the answer

Aluminium oxide, Al₂O₃

Al₂O₃ is amphoteric: it reacts with acids to give Al³⁺ salts and with NaOH to give aluminate ions. Na₂O and MgO are basic; SiO₂, P₄O₁₀ and SO₃ are acidic.

Dissolving and hydrolysing are different processes

The pH after adding water reveals more than whether the solid disappears.

Make a guess. You are not marked.

Sodium chloride and phosphorus(V) chloride are each added to water. How do the pH values compare?

NaCl gives about pH 7; PCl₅ gives about pH 2
Both give about pH 7, because both are chlorides
Both give about pH 2, because both contain chlorine

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NaCl gives about pH 7; PCl₅ gives about pH 2

NaCl is ionic and simply dissolves. PCl₅ is covalent and is hydrolysed, producing HCl and H₃PO₄, which make the solution strongly acidic.

Every specified chloride
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.

Check your understanding

A Period 3 chloride reacts with water to give an acidic solution and a white insoluble oxide. Which chloride is it?

PCl₅, which hydrolyses to give P₄O₁₀ as a solid
NaCl, which dissolves to give a neutral solution
SiCl₄, which hydrolyses to SiO₂ and HCl

Show the answer

SiCl₄, which hydrolyses to SiO₂ and HCl

SiCl₄ + 2H₂O -> SiO₂ + 4HCl. The white solid is SiO₂ and the HCl makes the solution strongly acidic.