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

This study guide covers Properties of Period 3 elements and their oxides and chlorides, section 3.2.4 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. The section has a single subsection, 3.2.4.1, and the specification lists it under International A2 inorganic chemistry, which makes everything here International A-level only. Unit 3 (Inorganic 2 and Physical 2) examines it, and any of it may also appear in Unit 5 (Practical and synoptic).

Pair it with the Period 3 oxides and chlorides revision notes and the Period 3 oxides and chlorides practice questions. See also the OxfordAQA A-Level Chemistry hub, the printable checklist and a free 10-minute diagnostic.

Background: Periodicity, Bonding and structure, Amount of substance and Acids and bases (pH).

What this unit covers

Every row is International A-level only.

Spec point	What you must be able to do
3.2.4.1	Describe how Na and Mg react with water
3.2.4.1	Describe the trends when Na to S react with oxygen and with chlorine
3.2.4.1	Explain the melting points of the oxides and chlorides from structure and bonding
3.2.4.1	Explain how the oxides and chlorides react with water, give the pH and write equations
3.2.4.1	Give the structures of the acids and anions formed from P₄O₁₀, SO₂, SO₃, SiCl₄ and PCl₅
3.2.4.1	Write equations for the oxides and chlorides with given acids and bases

No required practical is attached to this section.

The big idea: bonding changes across the period

Every trend here comes from one change. Sodium and magnesium have low electronegativity, so their oxides and chlorides are ionic. Silicon, phosphorus and sulfur are closer in electronegativity to oxygen and chlorine, so their compounds are covalent. Aluminium sits at the crossover: its small, highly charged ion distorts nearby anions, giving strong covalent character.

Sodium and magnesium with water

Sodium reacts fast with cold water: it floats, melts into a sphere and fizzes, leaving a colourless alkaline solution.

2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)

By contrast, magnesium barely reacts with cold water; a few hydrogen bubbles appear slowly on the surface. The magnesium hydroxide formed dissolves only slightly, so the solution is weakly alkaline.

Mg(s) + 2H₂O(l) → Mg(OH)₂(s) + H₂(g)

Heated magnesium burns in steam with a brilliant white light, forming the oxide.

Mg(s) + H₂O(g) → MgO(s) + H₂(g)

Sodium is the more reactive element because each atom loses one electron to form Na⁺. Each magnesium atom must lose two electrons, and removing the second one from an already positive ion costs extra energy.

Reactions of the elements with oxygen

Element	What you see	Equation
Na	orange flame, white solid	4Na + O₂ → 2Na₂O
Mg	intense white flame, white solid	2Mg + O₂ → 2MgO
Al	powder burns with white sparks	4Al + 3O₂ → 2Al₂O₃
Si	burns only on strong heating	Si + O₂ → SiO₂
P	white phosphorus catches fire, white solid forms	P₄ + 5O₂ → P₄O₁₀
S	pale blue flame, choking gas	S + O₂ → SO₂

Burning sulfur gives SO₂. Making SO₃ needs heat and a vanadium(V) oxide catalyst: 2SO₂ + O₂ ⇌ 2SO₃. The oxidation state in the highest oxide climbs from +1 (Na₂O) to +6 (SO₃), matching the number of outer-shell electrons.

Reactions of the elements with chlorine

Element	What you see	Equation	Product
Na	bright orange flame	2Na + Cl₂ → 2NaCl	white ionic solid
Mg	intense white flame	Mg + Cl₂ → MgCl₂	white ionic solid
Al	heated metal burns in a stream of dry chlorine	2Al + 3Cl₂ → Al₂Cl₆	very pale yellow solid that sublimes
Si	reacts on heating	Si + 2Cl₂ → SiCl₄	colourless liquid
P	catches fire in chlorine; excess chlorine gives the pentachloride	P₄ + 10Cl₂ → 4PCl₅	off-white solid

The specification limits the chlorides to these five, so no sulfur chloride is required. The oxidation state rises from +1 in NaCl to +5 in PCl₅.

Why aluminium chloride is Al₂Cl₆

In an AlCl₃ unit the aluminium atom has only six electrons in its outer shell. A chlorine atom on a second AlCl₃ unit donates a lone pair into this electron-deficient aluminium, forming a co-ordinate bond. Two such bonds join the units into an Al₂Cl₆ molecule with two bridging chlorine atoms, and each aluminium is then surrounded by four bonding pairs.

  Cl        Cl        Cl
     \    /    ↘    /
      Al          Al
     /    ↖    /    \
  Cl        Cl        Cl
(arrows: co-ordinate bonds from a bridging Cl to Al)

Worked example 1

Calculate the mass of Al₂Cl₆ formed when 0.540 g of aluminium reacts completely with chlorine. (Ar: Al = 27.0, Cl = 35.5)

2Al + 3Cl₂ → Al₂Cl₆
n(Al)     = 0.540 / 27.0 = 0.0200 mol
n(Al₂Cl₆) = 0.0200 / 2   = 0.0100 mol
Mr(Al₂Cl₆) = 2(27.0) + 6(35.5) = 267.0
mass       = 0.0100 × 267.0 = 2.67 g

Melting points of the oxides and chlorides

Oxides. From highest to lowest the order is MgO > Al₂O₃ > SiO₂ > Na₂O > P₄O₁₀ > SO₃, with SO₂ the lowest of all.

• Na₂O and MgO are giant ionic lattices. Mg²⁺ carries twice the charge of Na⁺ and is smaller, so the electrostatic attraction to O²⁻ is stronger and MgO melts higher.
• Al₂O₃ is still a giant lattice with high melting point, but Al³⁺ polarises the oxide ions so the bonding is partly covalent, and it melts below MgO.
• SiO₂ is macromolecular. Melting means breaking many strong Si–O covalent bonds.
• P₄O₁₀, SO₃ and SO₂ are simple molecules. Melting only overcomes van der Waals forces between molecules. The large P₄O₁₀ molecule has more electrons, so its van der Waals forces are the strongest of the three.

Chlorides. Typical values (°C):

Chloride	Melting behaviour	Structure
NaCl	melts at 801	giant ionic
MgCl₂	melts at 714	giant ionic, some covalent character
aluminium chloride	sublimes at about 180	Al₂Cl₆ molecules in the vapour
SiCl₄	melts at −69	simple molecular
PCl₅	turns to vapour at about 160 to 170	treated as simple molecular

Overcoming the ionic attraction in NaCl and MgCl₂ needs a lot of energy. MgCl₂ melts a little lower because the small Mg²⁺ ion polarises the large Cl⁻ ions, adding covalent character. Aluminium chloride sublimes at a low temperature: its bonding is largely covalent, and the vapour is Al₂Cl₆ molecules held together only by weak intermolecular forces. SiCl₄ is a liquid at room temperature: only van der Waals forces hold its molecules together. PCl₅ has more electrons per formula unit (100, against 82 in SiCl₄), so stronger van der Waals forces and a higher melting point. (In the solid PCl₅ actually exists as [PCl₄]⁺[PCl₆]⁻ ions; you are not asked about this.)

Worked example 2

Explain why magnesium chloride is a solid with a high melting point while silicon tetrachloride is a liquid at room temperature.

1. MgCl₂ is a giant ionic lattice with strong attraction between Mg²⁺ and Cl⁻ ions.
2. SiCl₄ is simple molecular. Only weak van der Waals forces between its molecules must be overcome; the Si–Cl bonds stay intact.
3. Far less energy is needed, so SiCl₄ melts at a much lower temperature.

Oxides with water

Oxide	Type	pH (approx.)	Equation
Na₂O	basic	14	Na₂O + H₂O → 2NaOH
MgO	basic	9	MgO + H₂O → Mg(OH)₂
Al₂O₃	amphoteric	7	insoluble, no reaction
SiO₂	acidic	7	insoluble, no reaction
P₄O₁₀	acidic	0	P₄O₁₀ + 6H₂O → 4H₃PO₄
SO₂	acidic	3	SO₂ + H₂O → H₂SO₃
SO₃	acidic	0	SO₃ + H₂O → H₂SO₄

Ionic oxides release oxide ions, which accept protons from water (O²⁻ + H₂O → 2OH⁻). NaOH is very soluble, but Mg(OH)₂ is not, so MgO gives the lower pH. Al₂O₃ and SiO₂ are giant structures that do not dissolve, so the pH stays at 7. Simple molecular oxides react with water to make acids. H₂SO₃ is a weak acid, only partly ionised, so it is less acidic than H₂SO₄ of equal concentration.

Chlorides with water

Chloride	What happens	Equation	pH (approx.)
NaCl	dissolves	NaCl(s) → Na⁺(aq) + Cl⁻(aq)	7
MgCl₂	dissolves	MgCl₂(s) → Mg²⁺(aq) + 2Cl⁻(aq)	6
AlCl₃	hisses; fumes of HCl with a little water	AlCl₃ + 6H₂O → [Al(H₂O)₆]³⁺ + 3Cl⁻	3
SiCl₄	violent reaction, white solid and steamy fumes	SiCl₄ + 2H₂O → SiO₂ + 4HCl	1
PCl₅	violent reaction, steamy fumes	PCl₅ + 4H₂O → H₃PO₄ + 5HCl	1

Explaining the trend. NaCl is ionic; its ions are simply hydrated, so the solution is neutral. MgCl₂ is ionic too, but the small Mg²⁺ ion pulls slightly on electron density in its water molecules, releasing a few H⁺ ions. The Al³⁺ ion is so small and highly charged that it strongly polarises the O–H bonds of its six water ligands:

[Al(H₂O)₆]³⁺ ⇌ [Al(H₂O)₅(OH)]²⁺ + H⁺

With very little water, aluminium chloride is fully hydrolysed instead: AlCl₃ + 3H₂O → Al(OH)₃ + 3HCl.

SiCl₄ and PCl₅ are covalent. Water attacks the central atom, the Si–Cl or P–Cl bonds break and hydrogen chloride forms, so the solutions are strongly acidic. With only a little cold water, PCl₅ stops at an intermediate: PCl₅ + H₂O → POCl₃ + 2HCl.

Worked example 3

A 3.40 g sample of SiCl₄ reacts fully with water, and the acidic mixture is diluted to 500 cm³. Find the HCl concentration, the pH and the mass of SiO₂ produced. (Ar: Si = 28.1, Cl = 35.5, O = 16.0)

Mr(SiCl₄) = 28.1 + 4(35.5) = 170.1
n(SiCl₄)  = 3.40 / 170.1 = 0.0200 mol
n(HCl)    = 4 × 0.0200   = 0.0800 mol
[HCl]     = 0.0800 / 0.500 = 0.160 mol dm⁻³
pH        = −log(0.160) = 0.80
mass SiO₂ = 0.0200 × 60.1 = 1.20 g

Structures of the acids and anions

• H₃PO₄ (from P₄O₁₀ or PCl₅): phosphorus at the centre with one P=O and three P–OH groups; tetrahedral. Losing protons gives H₂PO₄⁻, HPO₄²⁻ and finally PO₄³⁻, with the negative charges on single-bonded O.
• H₂SO₃ (from SO₂): sulfur with one S=O, two S–OH and a lone pair; pyramidal. Anions: HSO₃⁻ and SO₃²⁻, both keeping the lone pair.
• H₂SO₄ (from SO₃): sulfur with two S=O and two S–OH; tetrahedral. Anions: HSO₄⁻ and SO₄²⁻.
• HCl (from SiCl₄ and PCl₅): a single H–Cl bond; in water it gives H⁺ and Cl⁻.

HPO₄²⁻ (one O–H left)           HSO₃⁻ (lone pair on S)
        O                               O
        ‖                               ‖
  ⁻O –  P  – O⁻                   H–O – S – O⁻
        |                               ..
        O–H

Reactions with acids and bases

Basic oxides with acids: Na₂O + 2HNO₃ → 2NaNO₃ + H₂O and MgO + 2HCl → MgCl₂ + H₂O.

Amphoteric Al₂O₃: with acid, Al₂O₃ + 6HNO₃ → 2Al(NO₃)₃ + 3H₂O; with alkali, Al₂O₃ + 2KOH + 3H₂O → 2KAl(OH)₄.

Acidic oxides with bases: SiO₂ + Na₂O → Na₂SiO₃ (on heating), P₄O₁₀ + 12KOH → 4K₃PO₄ + 6H₂O, SO₂ + Ca(OH)₂ → CaSO₃ + H₂O and SO₃ + 2KOH → K₂SO₄ + H₂O.

Chlorides with alkali. The ionic chloride of magnesium gives a precipitate: MgCl₂ + 2NaOH → Mg(OH)₂ + 2NaCl. Aluminium chloride first gives white Al(OH)₃, which dissolves in excess alkali: AlCl₃ + 4NaOH → NaAl(OH)₄ + 3NaCl. The covalent chlorides are neutralised completely: SiCl₄ + 6NaOH → Na₂SiO₃ + 4NaCl + 3H₂O and PCl₅ + 8NaOH → Na₃PO₄ + 5NaCl + 4H₂O.

Common errors

• Writing AlCl₃ as the product of aluminium and chlorine. The specification asks for Al₂Cl₆.
• Saying SiCl₄ or PCl₅ “dissolves”. They react with water; the products are an acid and HCl.
• Giving pH 7 for MgCl₂ or AlCl₃. Only NaCl gives a neutral solution.
• Claiming that the low melting point of SiCl₄ comes from weak Si–Cl bonds. The bonds stay intact.
• Counting 3 mol of NaOH per mole of PCl₅. The five HCl molecules also need neutralising, giving 8.

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).