Period 3 Oxides and Chlorides

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.

Review acid–base definitions in your course’s Acids and Bases topic, then use its Periodic Table topic navigation for the complete 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.

Detailed Explanations

A. Highest oxidation number

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

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

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

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

Modelled example 1

Find Phosphorus Oxidation Numbers

Core

Problem

State the oxidation number of phosphorus in P₄O₁₀ and PCl₅.

Study the worked solution

1. 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.

2. 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.

Guided practice 2

Show that Aluminium Oxide Is Amphoteric

About 7 min

Problem

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

Try this before viewing the solution

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

1. 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.

2. Acid

   Method

   Write the neutralisation with hydrochloric acid.

   Reason

   The oxide behaves as a base toward acid.

   Working

   Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O.

3. 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.

Common misconception 3

Explain Covalent Aluminium Chloride

Find and correct the mistake

Learner claim

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

Try this before viewing the solution

Al³⁺ charge density

HighLow

Effect on chloride cloud

Strongly polarises itHas little effect

Check my attempt

View solution step by step

1. 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.

2. 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.

Challenge 4

Compare Sodium Chloride and Silicon Tetrachloride in Water

Minimal support

Bonding-to-reaction transfer

Compare the reactions of NaCl and SiCl₄ with water, including observations and an equation for hydrolysis.

Try this before viewing the solution

NaCl in water

Dissolves without hydrolysisHydrolyses vigorously

SiCl₄ in water

Hydrolyses vigorouslyNo reaction

Check my attempt

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

1. 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).

2. 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.

Common Mistakes

• Omitting oxidation numbers or assuming the oxide and chloride lists extend beyond the specified compounds.
• Calling every metal chloride ionic; AlCl₃ is the explicit exception.
• Saying every oxide reacts with water; Al₂O₃ and SiO₂ do not.
• Classifying Al₂O₃ or Al(OH)₃ as amphoteric without giving both an acid reaction and the sodium hydroxide reaction when asked.
• Writing AlCl₃ + 3H₂O → Al(OH)₃ + 3HCl as the inevitable result in excess water; the acidic hydrated aluminium ion remains important.

Use the topic check in Practise and check below after you can move from observation to structure, bonding and equation without skipping a link.

Exam Tips

• For “state and explain the variation”, give the numerical oxidation-number trend before explaining it.
• For bonding, name ionic lattice, giant covalent or molecular covalent as appropriate; then give the electronegativity or polarisation reason.
• For water reactions, distinguish dissolving, hydrolysis, no reaction and visible fumes/solid.
• For acid–base behaviour, one correct equation is evidence; an unsupported label is not.

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.