OxfordAQA A-Level Chemistry: Properties of Period 3 elements and their oxides and chlorides (9620) – Revision Notes

For full explanations and worked examples, start with the Period 3 oxides and chlorides study guide.

These notes summarise Properties of Period 3 elements and their oxides and chlorides, section 3.2.4 (subsection 3.2.4.1) of the OxfordAQA International AS and A-level Chemistry (9620) specification, Version 5.3, for International AS and A-level exams May/June 2020 onwards. Everything here is International A-level only (an International A2 section). Unit 3 (Inorganic 2 and Physical 2) tests it, and it can also come up in Unit 5 (Practical and synoptic). No required practical is attached to this section.

When you are ready to test yourself, use the Period 3 oxides and chlorides practice questions. Other topics are on the OxfordAQA A-Level Chemistry hub; tick off points on the printable checklist, and use the free 10-minute diagnostics to decide where to focus. Physical trends of the elements themselves are in the periodicity revision notes.

Definitions

• Basic oxide: neutralises an acid, giving a salt plus water (Na₂O, MgO).
• Acidic oxide: neutralises a base, giving a salt plus water (SiO₂, P₄O₁₀, SO₂, SO₃).
• Amphoteric: reacts with acids and with bases. Al₂O₃.
• Hydrolysis: a reaction in which water breaks bonds in a compound. SiCl₄, PCl₅, aluminium chloride.
• Co-ordinate bond: a shared pair of electrons that both come from one atom. Holds Al₂Cl₆ together.

Sodium and magnesium with water

Reaction	Key point	Equation
sodium in cold water	fast; floats, melts, fizzes; pH about 14	2Na + 2H₂O → 2NaOH + H₂
magnesium in cold water	slow, few bubbles; Mg(OH)₂ barely soluble; pH about 10	Mg + 2H₂O → Mg(OH)₂ + H₂
magnesium heated in steam	burns with white light; oxide, not hydroxide	Mg + H₂O → MgO + H₂

Na needs to lose one electron per atom; Mg needs to lose two, which costs far more energy.

Elements with oxygen and with chlorine

Element	With oxygen	With chlorine
Na	4Na + O₂ → 2Na₂O	2Na + Cl₂ → 2NaCl (orange flame)
Mg	2Mg + O₂ → 2MgO	Mg + Cl₂ → MgCl₂ (white flame)
Al	4Al + 3O₂ → 2Al₂O₃	2Al + 3Cl₂ → Al₂Cl₆ (pale yellow solid, sublimes)
Si	Si + O₂ → SiO₂	Si + 2Cl₂ → SiCl₄ (colourless liquid)
P	P₄ + 5O₂ → P₄O₁₀	P₄ + 10Cl₂ → 4PCl₅ (excess Cl₂; off-white solid)
S	S + O₂ → SO₂; then 2SO₂ + O₂ ⇌ 2SO₃ with V₂O₅	not required

Oxidation state in the highest oxide: +1, +2, +3, +4, +5, +6 from Na to S. In the chlorides it runs +1 to +5 from Na to P.

Al₂Cl₆ in one line: Al in AlCl₃ has only six outer electrons, so a Cl lone pair from a second unit forms a co-ordinate bond; two such bonds make the bridged dimer.

Structure and melting point

Compound	Structure	What melting overcomes	Melting point
Na₂O, MgO	giant ionic	ionic attraction	high (MgO highest of all)
Al₂O₃	giant ionic, partly covalent	ionic attraction	high, below MgO
SiO₂	macromolecular	covalent bonds	high
P₄O₁₀, SO₃, SO₂	simple molecular	van der Waals forces	low
NaCl (801 °C), MgCl₂ (714 °C)	giant ionic	ionic attraction	high
aluminium chloride	molecular Al₂Cl₆ when vaporised	weak intermolecular forces	sublimes about 180 °C
SiCl₄ (−69 °C)	simple molecular	van der Waals forces	low
PCl₅	treated as simple molecular	van der Waals forces	vaporises about 160 to 170 °C

Oxides, lowest to highest melting point: SO₂ < SO₃ < P₄O₁₀ < Na₂O < SiO₂ < Al₂O₃ < MgO.

Method in steps: comparing two melting points

1. Name each structure.
2. Name the force or bond overcome in each.
3. Say which is stronger and why (ion charge and size; number of electrons; covalent bonds versus intermolecular forces).
4. Link stronger forces to more energy needed, so higher melting point.

Oxides with water

Oxide	pH	Why	Equation
Na₂O	14	ionic; O²⁻ + H₂O → 2OH⁻	Na₂O + H₂O → 2NaOH
MgO	9	ionic, but hydroxide sparingly soluble	MgO + H₂O → Mg(OH)₂
Al₂O₃	7	giant lattice, does not dissolve	no reaction
SiO₂	7	macromolecular, does not dissolve	no reaction
P₄O₁₀	0	simple molecular; forms acid	P₄O₁₀ + 6H₂O → 4H₃PO₄
SO₂	3	weak acid, partly ionised	SO₂ + H₂O → H₂SO₃
SO₃	0	strong acid	SO₃ + H₂O → H₂SO₄

Chlorides with water

Chloride	Observation	Equation	pH
NaCl	dissolves	NaCl → Na⁺(aq) + Cl⁻(aq)	7
MgCl₂	dissolves	MgCl₂ → Mg²⁺(aq) + 2Cl⁻(aq)	6
AlCl₃	hisses, steamy fumes	AlCl₃ + 6H₂O → [Al(H₂O)₆]³⁺ + 3Cl⁻	3
SiCl₄	violent; white solid and fumes	SiCl₄ + 2H₂O → SiO₂ + 4HCl	1
PCl₅	violent; fumes	PCl₅ + 4H₂O → H₃PO₄ + 5HCl	1

• Acidity of hydrated aluminium: [Al(H₂O)₆]³⁺ ⇌ [Al(H₂O)₅(OH)]²⁺ + H⁺, because the small, 3+ ion polarises the O–H bonds of its water ligands.
• With little water: AlCl₃ + 3H₂O → Al(OH)₃ + 3HCl and PCl₅ + H₂O → POCl₃ + 2HCl.
• The pattern: ionic chlorides dissolve; covalent chlorides are hydrolysed and release HCl.

Structures of acids and anions

Formed from	Acid	Anions	Shape at P or S
P₄O₁₀, PCl₅	H₃PO₄ (P=O, three P–OH)	H₂PO₄⁻, HPO₄²⁻, PO₄³⁻	tetrahedral
SO₂	H₂SO₃ (S=O, two S–OH, lone pair)	HSO₃⁻, SO₃²⁻	pyramidal
SO₃	H₂SO₄ (two S=O, two S–OH)	HSO₄⁻, SO₄²⁻	tetrahedral
SiCl₄, PCl₅	HCl	Cl⁻	not applicable

Negative charges sit on single-bonded oxygen atoms. Each proton lost turns one O–H into O⁻.

Acid and base equations

Substance	Reagent	Equation
Na₂O	HNO₃	Na₂O + 2HNO₃ → 2NaNO₃ + H₂O
MgO	HCl	MgO + 2HCl → MgCl₂ + H₂O
Al₂O₃	HNO₃	Al₂O₃ + 6HNO₃ → 2Al(NO₃)₃ + 3H₂O
Al₂O₃	KOH	Al₂O₃ + 2KOH + 3H₂O → 2KAl(OH)₄
SiO₂	Na₂O, heat	SiO₂ + Na₂O → Na₂SiO₃
P₄O₁₀	KOH	P₄O₁₀ + 12KOH → 4K₃PO₄ + 6H₂O
SO₂	Ca(OH)₂	SO₂ + Ca(OH)₂ → CaSO₃ + H₂O
MgCl₂	NaOH	MgCl₂ + 2NaOH → Mg(OH)₂ + 2NaCl
AlCl₃	excess NaOH	AlCl₃ + 4NaOH → NaAl(OH)₄ + 3NaCl
SiCl₄	NaOH	SiCl₄ + 6NaOH → Na₂SiO₃ + 4NaCl + 3H₂O
PCl₅	NaOH	PCl₅ + 8NaOH → Na₃PO₄ + 5NaCl + 4H₂O

Method in steps: chloride plus alkali

1. Write the hydrolysis products (for PCl₅: H₃PO₄ + 5HCl).
2. Count the acidic hydrogens (3 + 5 = 8).
3. Use that many OH⁻; the products are the sodium salts plus water.

Must-know distinctions

• AlCl₃ vs Al₂Cl₆: the element reaction gives Al₂Cl₆; AlCl₃ is the formula used for its reaction with water.
• Dissolves vs hydrolysed: NaCl and MgCl₂ dissolve; SiCl₄ and PCl₅ react with water.
• Insoluble vs neutral: Al₂O₃ and SiO₂ leave water at pH 7, yet one is amphoteric and one is acidic.
• H₂SO₃ vs H₂SO₄: S stays +4 from SO₂ and +6 from SO₃.
• Mg + cold water vs steam: hydroxide vs oxide.

Quick self-test

1. Write the equation for aluminium reacting with chlorine, as the specification requires.
2. Give the pH of magnesium chloride solution, to the nearest whole number.
3. Which Period 3 chloride listed in the specification is a liquid at room temperature?
4. Explain in one sentence why SiCl₄ has a low melting point.
5. Write the full hydrolysis equation for PCl₅.
6. Name the bond type that joins the two AlCl₃ units in Al₂Cl₆.
7. How many moles of HCl form when 0.040 mol of PCl₅ is fully hydrolysed?
8. What mass of MgCl₂ forms when 0.486 g of Mg burns in chlorine? (Ar: Mg = 24.3, Cl = 35.5)
9. How many moles of Cl₂ react with 0.050 mol of P₄ to form PCl₅?
10. How many moles of NaOH neutralise 0.015 mol of SiCl₄ to give Na₂SiO₃?
11. State the shape around sulfur in HSO₃⁻.
12. Why does AlCl₃ solution have a lower pH than MgCl₂ solution?

Answers

1. 2Al + 3Cl₂ → Al₂Cl₆
2. About 6.
3. SiCl₄.
4. Its molecules are held together only by weak van der Waals forces, which need little energy to overcome.
5. PCl₅ + 4H₂O → H₃PO₄ + 5HCl
6. Co-ordinate (dative covalent) bonds.
7. 5 × 0.040 = 0.20 mol.
8. 0.486 / 24.3 = 0.0200 mol; Mr MgCl₂ = 95.3; mass = 1.91 g.
9. 10 × 0.050 = 0.50 mol.
10. 6 × 0.015 = 0.090 mol.
11. Pyramidal (three bonding regions and one lone pair).
12. The Al³⁺ ion has a higher charge density than Mg²⁺, so its water ligands are polarised more and more H⁺ is released.

Where marks are usually lost

• Writing 2Al + 3Cl₂ → 2AlCl₃; the specification lists Al₂Cl₆ as the product.
• Describing SiCl₄ or PCl₅ as “dissolving to give a neutral solution”.
• Leaving out the steamy fumes of HCl when describing covalent chloride hydrolysis.
• Giving MgCl₂ the same pH as NaCl without comment.
• Saying Si–Cl or P–Cl bonds break when the chlorides melt.
• Using 3 mol NaOH per mole of PCl₅ instead of 8.
• Writing MgO as the product of magnesium with cold water.
• Calling Al₂O₃ a “neutral oxide” because water stays at pH 7.
• Missing sulfur’s lone pair in H₂SO₃, HSO₃⁻ or SO₃²⁻.

Official syllabus

OxfordAQA, International AS and A-level Chemistry (9620) specification, Version 5.3, for International AS and A-level exams May/June 2020 onwards: section 3.2.4 Properties of Period 3 elements and their oxides and chlorides (International A2).