AQA A-Level Chemistry: Group 2, the alkaline earth metals (7405) – Revision Notes

These notes condense section 3.2.2 Group 2, the alkaline earth metals of the AQA AS and A-level Chemistry specification (7404/7405), version 1.2, July 2026, for AS and A-level exams June 2016 onwards. Everything here is AS content (nothing is A-level only). It is examined on AS Paper 1 and A-level Paper 1, and A-level Paper 3 can test any content. For full explanations and worked examples, read the Group 2 study guide first.

When you are ready, test yourself with the Group 2 practice questions. The course hub is AQA A-Level Chemistry, the printable checklist lists every specification point, and the free 10-minute diagnostics will show you where to start. The Period 3 versions of the same trends are in the periodicity revision notes.

1. Key facts

• Group 2 elements (Mg, Ca, Sr, Ba in this topic) have two outer electrons in an s sub-shell: s block.
• In compounds they form M²⁺ ions, oxidation state +2.
• All four are giant metallic lattices with metallic bonding.

2. Physical trends, Mg to Ba

Property	Trend down the group	Reason
Atomic radius	increases	one more occupied shell each time
First ionisation energy	decreases	outer electron further away and more shielded, so attracted less strongly
Melting point	generally decreases (Mg is an exception, lower than Ca)	larger ions, same 2+ charge, so weaker attraction between ions and delocalised electrons

Method: explaining an ionisation energy trend down a group

1. Distance:   outer electron is in a higher shell, further from the nucleus.
2. Shielding:  more inner shells, more shielding.
3. Attraction: weaker attraction, so less energy needed to remove the electron.
4. Nuclear charge rises, but 1 and 2 outweigh it.

Equation for a first ionisation energy (state symbols needed): Sr(g) → Sr⁺(g) + e⁻

Method: explaining the melting point trend (Ca to Ba)

1. Structure:  giant metallic lattice, metallic bonding.
2. Same charge: each ion is 2+, two delocalised electrons per atom.
3. Size:        ions get larger down the group.
4. Result:      weaker attraction between ions and delocalised electrons, less energy to overcome.

Magnesium has a different crystal structure (hexagonal close-packed), so it does not follow this pattern.

3. Reactions with water

Reaction	Equation	Observations
Ca, Sr, Ba with cold water	M + 2H₂O → M(OH)₂ + H₂	fizzing, metal dissolves, alkaline solution; faster down the group
Mg with cold water	Mg + 2H₂O → Mg(OH)₂ + H₂	very slow
Mg with steam	Mg + H₂O → MgO + H₂	burns with a bright white light, white solid

• Reactivity increases down the group because the outer electrons are lost more easily (lower ionisation energies).
• Each reaction is redox: metal 0 → +2; hydrogen +1 → 0.

4. Uses of Group 2 metals and compounds

Substance	Use	Equation to know
Mg	reducing agent in titanium extraction	TiCl₄ + 2Mg → Ti + 2MgCl₂
Mg(OH)₂	antacid in medicine	Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O
Ca(OH)₂	raises the pH of acidic soil	Ca(OH)₂ + 2H⁺ → Ca²⁺ + 2H₂O
CaO	removes SO₂ from flue gases	CaO + SO₂ → CaSO₃
CaCO₃	removes SO₂ from flue gases	CaCO₃ + SO₂ → CaSO₃ + CO₂
BaSO₄	barium meal for X-ray imaging	(insoluble, so Ba²⁺ is not absorbed)

Worked reminder: titanium atom economy. Using Aᵣ Ti = 47.9, Mg = 24.3, Cl = 35.5:

Mr(TiCl₄) = 189.9      total mass of reactants = 189.9 + (2 × 24.3) = 238.5
atom economy for Ti = 47.9 ÷ 238.5 × 100 = 20.1%

Worked reminder: an antacid tablet. A tablet holds 0.500 g of Mg(OH)₂ (Mr 58.3).

n(Mg(OH)₂) = 0.500 ÷ 58.3 = 8.58 × 10⁻³ mol
n(HCl) neutralised = 2 × 8.58 × 10⁻³ = 1.72 × 10⁻² mol

Worked reminder: flue gas. A plant must remove 128.2 kg of SO₂ (Mr 64.1).

n(SO₂) = 128.2 × 10³ ÷ 64.1 = 2000 mol
CaO route:   n(CaO) = 2000 mol    mass = 2000 × 56.1  = 112.2 kg
CaCO₃ route: n(CaCO₃) = 2000 mol  mass = 2000 × 100.1 = 200.2 kg, and 2000 mol of CO₂ (88.0 kg) is released

Every route has a 1 : 1 ratio of base to SO₂, so the amount in moles of base always equals the amount of SO₂.

5. Solubility trends

Compound type	Trend down the group	Extremes the specification names
Hydroxides, M(OH)₂	solubility increases	Mg(OH)₂ is sparingly soluble
Sulfates, MSO₄	solubility decreases	BaSO₄ is insoluble

Memory hook: the two trends run in opposite directions. Hydroxides go up; sulfates go down.

A saturated solution of a hydroxide lower in the group contains more OH⁻ ions, so it has a higher pH.

6. The test for sulfate ions

1. Add dilute HCl (or HNO₃) to the solution.
2. Add BaCl₂ solution.
3. White precipitate = sulfate present:  Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)

• BaCl₂ is used because BaSO₄ is insoluble, so it precipitates.
• The acid is added to remove carbonate (and sulfite) ions, which would also give a white precipitate with Ba²⁺: 2H⁺ + CO₃²⁻ → CO₂ + H₂O.
• Never use H₂SO₄ to acidify: it adds sulfate ions.

7. Required practical 4: Group 2 cations

Required practical 4 is listed in section 3.2.3 and includes identifying Group 2 cations.

Reagent added	Mg²⁺	Ca²⁺	Sr²⁺	Ba²⁺
NaOH(aq)	most precipitate	less	little	least or none
dilute H₂SO₄	none	slight	more	most precipitate

Method: identifying an unknown Group 2 solution

1. Add NaOH(aq): a heavy white precipitate points to Mg²⁺; little or none points to Ba²⁺.
2. Add dilute H₂SO₄ to a fresh sample: a white precipitate points to Ba²⁺; none points to Mg²⁺.
3. Use both results together; Ca²⁺ and Sr²⁺ give intermediate results.

8. Must-know distinctions

• Cold water vs steam (Mg): hydroxide with cold water; oxide with steam.
• Sparingly soluble vs insoluble: Mg(OH)₂ is sparingly soluble; BaSO₄ is insoluble.
• CaO vs CaCO₃ in flue gases: both give CaSO₃; only CaCO₃ releases CO₂.
• Ba²⁺ vs BaSO₄: barium ions are toxic; barium sulfate is safe to swallow because it does not dissolve.
• Oxidising vs reducing agent: in the titanium extraction Mg is oxidised, so it is the reducing agent.

Quick self-test

1. State and explain the trend in atomic radius from Mg to Ba.
2. Write an equation, with state symbols, for the first ionisation energy of barium.
3. Write an equation for the reaction of strontium with water.
4. Name the products when magnesium reacts with steam.
5. Which Group 2 hydroxide is the most soluble?
6. Which Group 2 sulfate is the most soluble?
7. Give one use of calcium hydroxide and one use of magnesium hydroxide.
8. Why is the solution acidified before adding barium chloride in the sulfate test?
9. Why is barium sulfate safe to use in a barium meal?
10. 4.86 g of magnesium reacts completely with excess TiCl₄. Calculate the mass of titanium formed. (Aᵣ Mg = 24.3, Ti = 47.9)
11. Calculate the amount, in moles, of hydrogen formed when 2.00 g of calcium reacts with excess water. (Aᵣ Ca = 40.1)
12. Give one disadvantage of using CaCO₃ rather than CaO to remove SO₂.

Answers

1. Increases; each element has an extra occupied shell, so the outer electrons are further from the nucleus.
2. Ba(g) → Ba⁺(g) + e⁻
3. Sr + 2H₂O → Sr(OH)₂ + H₂
4. Magnesium oxide and hydrogen.
5. Barium hydroxide, Ba(OH)₂.
6. Magnesium sulfate, MgSO₄.
7. Ca(OH)₂ raises the pH of acidic soil; Mg(OH)₂ is an antacid (neutralises excess stomach acid).
8. To remove carbonate (and sulfite) ions, which would also form a white precipitate with barium ions.
9. It is insoluble, so toxic barium ions are not absorbed into the blood.
10. n(Mg) = 4.86 ÷ 24.3 = 0.200 mol; n(Ti) = 0.100 mol; mass = 0.100 × 47.9 = 4.79 g.
11. n(Ca) = 2.00 ÷ 40.1 = 0.0499 mol; ratio 1 : 1, so 0.0499 mol of H₂.
12. It releases carbon dioxide, a greenhouse gas.

Where marks are usually lost

• Explaining the fall in ionisation energy without mentioning shielding, or claiming nuclear charge decreases down the group.
• Giving MgO as the product of magnesium with cold water, or Mg(OH)₂ with steam.
• Leaving state symbols off an ionisation energy equation, or writing the metal as (s).
• Swapping the hydroxide and sulfate trends.
• Saying “melting point decreases because the atoms get bigger” without describing the metallic bonding (ions and delocalised electrons).
• In the sulfate test, saying the acid is added “to make the precipitate form” instead of to remove carbonate ions.
• Saying barium sulfate is used because barium “is not poisonous”.
• Calling magnesium an oxidising agent in the titanium extraction, or getting the TiCl₄ : Mg ratio wrong (it is 1 : 2).

Official syllabus

AQA, AS and A-level Chemistry specification (7404/7405), version 1.2, July 2026, for AS and A-level exams June 2016 onwards: section 3.2.2 Group 2, the alkaline earth metals, with required practical 4 (listed in section 3.2.3).