Period 3 Oxides and Chlorides

Learn and apply Period 3 Oxides and Chlorides in the published Chemistry course sequence.

GCE A-Level H2 Chemistry 9476-2027

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Period 3 Oxides and Chlorides: Orientation

Period 3 chemistry questions reward a fixed sequence: identify oxidation number and bonding, describe the reaction with water, then prove acid–base behaviour with the requested equation.

Use the Acids and Bases theories lesson for acid–base definitions and the Periodic Table hub for the complete topic sequence.

Definitions (Must Know)

A. Highest oxidation number

The highest oxidation number is the most positive oxidation state shown by the Period 3 element in the specified oxide or chloride.

B. Basic, acidic and amphoteric

A basic oxide or hydroxide reacts with an acid to form salt and water.
An acidic oxide reacts with a base to form salt and water.
An amphoteric oxide or hydroxide reacts with both acids and bases.

C. Hydrolysis

Hydrolysis is reaction with water. Covalent Period 3 chlorides can hydrolyse to produce hydrogen chloride; hydrated metal ions can also transfer a proton to water and make a solution acidic.

Key Ideas (What Earns Marks)

The highest oxidation number rises from +1 in Na₂O/NaCl to +6 in SO₃; the specified chloride series ends at +5 in PCl₅.
Across the specified oxides and chlorides, bonding generally changes from ionic to covalent as the electronegativity difference decreases.
AlCl₃ is the named exception to a simple electronegativity argument: the small, highly charged Al³⁺ strongly polarises chloride electron clouds, giving predominantly covalent bonding.
Oxide character changes from basic through amphoteric to acidic.
State “no reaction with water” where appropriate; do not invent an equation for Al₂O₃ or SiO₂ with water.
The specified hydroxides are NaOH, Mg(OH)₂ and Al(OH)₃; aluminium hydroxide is amphoteric and its base reaction is required with sodium hydroxide only.

Oxides: Na₂O, MgO, Al₂O₃, SiO₂, P₄O₁₀, SO₃.
Chlorides: NaCl, MgCl₂, AlCl₃, SiCl₄, PCl₅.
Hydroxides: NaOH, Mg(OH)₂, Al(OH)₃.

Period 3 chemistry map: oxidation number rises across the specified compounds, oxide character changes from basic to acidic, and chloride reactions with water depend on bonding and hydrolysis.

Detailed Explanations

A. Highest oxidation number

Oxygen is assigned -2 and chlorine -1 in these compounds:

Element	Specified oxide	Oxidation number	Specified chloride	Oxidation number
Na	Na₂O	+ 1	NaCl	+ 1
Mg	MgO	+ 2	MgCl₂	+ 2
Al	Al₂O₃	+ 3	AlCl₃	+ 3
Si	SiO₂	+ 4	SiCl₄	+ 4
P	P₄O₁₀	+ 5	PCl₅	+ 5
S	SO₃	+ 6	—	—

The rise reflects the increasing number of valence electrons available for bonding across Period 3.

B. Bonding and structure

Na₂O and MgO are predominantly ionic; Al₂O₃ is an ionic lattice with appreciable covalent character.
SiO₂ is giant covalent; P₄O₁₀ and SO₃ are covalent molecular oxides.
NaCl and MgCl₂ form ionic lattices.
AlCl₃ is predominantly covalent because of strong polarisation by Al³⁺ and forms dimeric Al₂Cl₆ units under relevant molecular conditions.
SiCl₄ is molecular covalent. The P–Cl bonds in PCl₅ are covalent; in the solid state it is commonly represented as ionic [PCl₄] + [PCl₆]⁻, so state conditions matter when naming its structure.

Do not explain AlCl₃ solely by electronegativity difference; the syllabus explicitly treats it as the exception.

C. Oxides with water and acid–base behaviour

Oxide	Reaction with water	Acid–base character
Na₂O	reacts readily: Na₂O + H₂O → 2NaOH	basic
MgO	reacts slowly; sparingly soluble Mg(OH)₂ forms	basic
Al₂O₃	no reaction	amphoteric
SiO₂	no reaction	acidic; reacts with hot concentrated NaOH
P₄O₁₀	P₄O₁₀ + 6H₂O → 4H₃PO₄	acidic
SO₃	SO₃ + H₂O → H₂SO₄	acidic

Proof equations for amphoteric Al₂O₃:

Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O

Al₂O₃ + 2NaOH + 3H₂O → 2NaAl(OH)₄

An acidic-oxide proof:

SiO₂ + 2NaOH → Na₂SiO₃ + H₂O

Use hot concentrated sodium hydroxide for the slow reaction with SiO₂.

D. The specified hydroxides

NaOH is a soluble strong base.
Mg(OH)₂ is a sparingly soluble base.
Al(OH)₃ is amphoteric:

Al(OH)₃ + 3HCl → AlCl₃ + 3H₂O

Al(OH)₃ + NaOH → NaAl(OH)₄

The second equation uses sodium hydroxide, matching the stated syllabus scope.

E. Chlorides with water

Chloride	What happens with water
NaCl	dissolves to give an approximately neutral solution; no hydrolysis
MgCl₂	dissolves; hydrated Mg²⁺ undergoes slight hydrolysis, giving a mildly acidic solution
AlCl₃	dissolves/hydrolyses to give a distinctly acidic solution; anhydrous material fumes in moist air
SiCl₄	vigorous hydrolysis gives steamy HCl fumes and hydrated silica/white SiO₂
PCl₅	vigorous, stepwise hydrolysis ultimately gives H₃PO₄ and HCl

For aluminium chloride in excess water, acidity is better represented by hydrolysis of the hydrated ion than by claiming that Al(OH)₃ necessarily precipitates:

[Al(H₂O)₆]³⁺ + H₂O ⇌ [Al(H₂O)₅(OH)]²⁺ + H₃O⁺

Overall equations for the covalent non-metal chlorides:

SiCl₄ + 2H₂O → SiO₂ + 4HCl

PCl₅ + 4H₂O → H₃PO₄ + 5HCl

Worked Examples

Problem

State the oxidation number of phosphorus in

P₄O₁₀ and

PCl₅.

Study the worked solution

Use oxygen in the oxide
Method
Set each oxygen to -2 and make the neutral formula sum to zero.
Reason
Ten oxygen atoms contribute -20, so four phosphorus atoms must contribute + 20.
Working
4x + 10(-2) = 0 ⇒ x = +5.
Use chlorine in the chloride
Method
Set each chlorine to -1.
Reason
Five chlorine atoms contribute -5 in a neutral molecule.
Working
x + 5(-1) = 0 ⇒ x = +5.

Problem

Describe the reactions of

Al₂O₃ with water, hydrochloric acid and aqueous sodium hydroxide, including equations where reaction occurs.

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Water observation, two equations and classification

Hints

Hint 1: water and acid

There is no reaction with water; with acid, form aluminium chloride and water.

Hint 2: alkali product

In aqueous sodium hydroxide, form soluble tetrahydroxoaluminate.

View solution step by step

Water
Method
State no reaction with water.
Reason
Aluminium oxide does not simply form a hydroxide solution on contact with water.
Working
Al₂O₃: no reaction with H₂O.
Acid
Method
Write the neutralisation with hydrochloric acid.
Reason
The oxide behaves as a base toward acid.
Working
Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O.
Alkali and classification
Method
Form aqueous tetrahydroxoaluminate and call the oxide amphoteric.
Reason
Reaction with both acid and base is the required evidence for amphoterism.
Working
Al₂O₃ + 2NaOH + 3H₂O → 2NaAl(OH)₄.

Learner claim

A learner says

AlCl₃ must be ionic because aluminium is a metal and chlorine is a non-metal. Correct the bonding explanation.

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Al³⁺ charge density

HighLow

Effect on chloride cloud

Strongly polarises itHas little effect

View solution step by step

Describe the cation
Method
Identify Al³⁺ as small and highly charged.
Reason
Its high charge density creates a strong electric field.
Working
Small radius; charge + 3.
Link polarisation to bonding
Method
State that it strongly distorts the Cl⁻ electron cloud.
Reason
Strong polarisation gives the Al–Cl bonds substantial covalent character.
Working
AlCl₃ is predominantly covalent.

Bonding-to-reaction transfer

Compare the reactions of

NaCl and

SiCl₄ with water, including observations and an equation for hydrolysis.

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NaCl in water

Dissolves without hydrolysisHydrolyses vigorously

SiCl₄ in water

Hydrolyses vigorouslyNo reaction

Hints

Hint 1: products

Silicon tetrachloride produces hydrogen chloride and hydrated silica.

Hint 2: balance

Four chlorine atoms require four

HCl molecules.

View solution step by step

Ionic chloride
Method
State that sodium chloride dissolves into hydrated ions without hydrolysis.
Reason
Its solution is approximately neutral.
Working
NaCl(s) → Na + (aq) + Cl-(aq).
Covalent chloride
Method
Describe vigorous hydrolysis, steamy hydrogen chloride fumes and white hydrated silica.
Reason
The molecular chloride reacts with water rather than merely dissociating.
Working
SiCl₄ + 2H₂O → SiO₂ + 4HCl.

Next step

Try the unknown silicon-compound Mind Stretcher independently.

Mind Stretchers

Mind stretcher 1Extension

An unknown Period 3 element forms a giant covalent acidic oxide that does not react with water and a molecular tetrachloride that hydrolyses vigorously. Identify the element and justify every clue.

Show Answer

Mark scheme:

The giant covalent acidic oxide that does not react with water is SiO₂.
A Period 3 tetrachloride is SiCl₄, consistent with silicon in oxidation state + 4.
SiCl₄ hydrolyses: SiCl₄ + 2H₂O → SiO₂ + 4HCl.
Therefore the element is silicon.

Period 3 Oxides and Chlorides

Continue where you stopped

Period 3 Oxides and Chlorides: Orientation

Definitions (Must Know)

A. Highest oxidation number

B. Basic, acidic and amphoteric

C. Hydrolysis

Detailed Explanations

A. Highest oxidation number

B. Bonding and structure

C. Oxides with water and acid–base behaviour

D. The specified hydroxides

E. Chlorides with water

Worked Examples

Find Phosphorus Oxidation Numbers

Use oxygen in the oxide

Use chlorine in the chloride

Show that Aluminium Oxide Is Amphoteric

Water

Acid

Alkali and classification

Explain Covalent Aluminium Chloride

Describe the cation

Link polarisation to bonding

Compare Sodium Chloride and Silicon Tetrachloride in Water

Ionic chloride

Covalent chloride

Mind Stretchers

Mind stretcher 1Extension