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

This guide teaches 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. It covers every content point in that section, from the physical trends of Mg to Ba through to the test for sulfate ions. All of it is AS content, so nothing here is A-level only. The specification places 3.2.2 on AS Paper 1 (inorganic sections 3.2.1 to 3.2.3) and on A-level Paper 1 (all of section 3.2), and A-level Paper 3 can test any content.

For quick recall, use the Group 2 revision notes. To test yourself, use the Group 2 practice questions. The course hub is AQA A-Level Chemistry, and the printable checklist lists every specification point. To find your weak spots first, try one of the free 10-minute diagnostics.

This topic builds on three earlier ones. The trend explanations use the same nuclear charge, shielding and distance ideas as periodicity. How successive ionisation energies in Group 2 give evidence for shells is in atomic structure. Oxidation states and half-equations are in oxidation, reduction and redox equations.

What this unit covers

Spec point (3.2.2)	What you must be able to do	Status
Physical trends	Explain the trends in atomic radius and first ionisation energy, Mg–Ba	AS and A-level
Physical trends	Explain the melting points in terms of structure and bonding	AS and A-level
Reactions with water	Describe the reactions of Mg–Ba with water (and Mg with steam)	AS and A-level
Titanium	Describe the use of magnesium to extract titanium from TiCl₄	AS and A-level
Hydroxides	State the relative solubilities of the hydroxides Mg–Ba; Mg(OH)₂ is sparingly soluble	AS and A-level
Uses	Mg(OH)₂ in medicine, Ca(OH)₂ in agriculture, CaO or CaCO₃ to remove SO₂ from flue gases	AS and A-level
Sulfates	State the relative solubilities of the sulfates Mg–Ba; BaSO₄ is insoluble	AS and A-level
Sulfate test	Explain why BaCl₂ solution is used to test for sulfate ions and why it is acidified	AS and A-level
Medicine	Describe the use of BaSO₄ in medicine	AS and A-level

Required practical 4 (test-tube reactions to identify cations, including Group 2, and anions, including SO₄²⁻) is listed at the end of section 3.2.3 but uses this topic directly, so it is covered below.

Group 2 at a glance

Element	Z	Outer electrons	Atomic radius trend	First IE (kJ mol⁻¹, approx.)	Melting point (°C, approx.)
Mg	12	3s²	smallest	738	650
Ca	20	4s²	↓ increases	590	842
Sr	38	5s²	↓	550	777
Ba	56	6s²	largest	503	727

Every element has two outer electrons in an s sub-shell, so Group 2 is in the s block. In compounds the metals form 2+ ions and have oxidation state +2.

Atomic radius

Trend: atomic radius increases down the group.

Explanation: each element down the group has one more occupied shell. The outer electrons are further from the nucleus. The extra protons are offset by extra shielding from the additional inner shell.

First ionisation energy

The first ionisation energy of calcium is the energy for:

Ca(g) → Ca⁺(g) + e⁻

Trend: first ionisation energy decreases down the group.

Explanation, in three linked points:

1. The outer electron is in a shell further from the nucleus (larger atomic radius).
2. There is more shielding from inner shells.
3. So the attraction between the nucleus and the outer electron is weaker, and less energy is needed to remove it. The increase in nuclear charge is outweighed by the extra distance and shielding.

Worked example 1: comparing two ionisation energies

Explain why barium has a lower first ionisation energy than calcium.

• Barium’s outer electron is in the sixth shell; calcium’s is in the fourth. Barium’s outer electron is further from the nucleus.
• Barium has more inner shells, so more shielding.
• The outer electron in barium is attracted less strongly, so less energy is needed to remove it.

Add that distance and shielding outweigh barium’s larger nuclear charge.

Melting point

All four elements are giant metallic lattices. Metallic bonding is the attraction between the positive ions and the delocalised electrons.

Trend: melting point generally decreases from Ca to Ba.

Explanation:

• Each element forms 2+ ions and gives two delocalised electrons per atom, so the charge stays the same.
• The ions get larger down the group.
• The delocalised electrons are further from the nucleus of each ion, so the attraction between the ions and the delocalised electrons is weaker.
• Less energy is needed to overcome the metallic bonding.

Magnesium does not fit: its melting point is lower than calcium’s. Magnesium has a different metallic crystal structure (hexagonal close-packed; calcium is face-centred cubic), so the simple charge and size argument does not apply to it. Use the argument for Ca, Sr and Ba, and treat Mg as the exception.

Reactions with water

Reactivity with water increases down the group. The reason links back to ionisation energy: each metal loses its two outer electrons when it reacts, and these are lost more easily further down the group.

Metal	Reaction
Mg	reacts very slowly with cold water; reacts with steam
Ca	reacts steadily with cold water: fizzing, the metal dissolves, a white suspension may form
Sr	reacts faster than calcium
Ba	reacts most vigorously

Cold water (Ca, Sr, Ba, and very slowly Mg) gives the hydroxide and hydrogen:

Ca(s) + 2H₂O(l) → Ca(OH)₂(aq) + H₂(g)

The general form is M + 2H₂O → M(OH)₂ + H₂. The solution formed is alkaline. With calcium some calcium hydroxide may come out as a white solid, because it is only slightly soluble.

Steam (Mg) gives the oxide, not the hydroxide:

Mg(s) + H₂O(g) → MgO(s) + H₂(g)

Magnesium burns in steam with a bright white light, leaving white MgO.

Both reactions are redox. The metal is oxidised from 0 to +2; hydrogen is reduced from +1 to 0.

The extraction of titanium using magnesium

Titanium is extracted from titanium(IV) chloride by reduction with magnesium:

TiCl₄ + 2Mg → Ti + 2MgCl₂

Magnesium is the reducing agent: it is oxidised from 0 to +2, and titanium is reduced from +4 to 0.

Worked example 2: masses in the titanium extraction

Calculate the mass of magnesium needed to produce 75.0 kg of titanium, and the mass of TiCl₄ used. (Aᵣ: Ti = 47.9, Mg = 24.3, Cl = 35.5)

n(Ti)     = 75.0 × 10³ g ÷ 47.9           = 1566 mol (1565.8)
n(Mg)     = 2 × n(Ti)                       = 3132 mol
mass Mg   = 3131.5 × 24.3 = 76.1 × 10³ g    = 76.1 kg
Mr(TiCl₄) = 47.9 + (4 × 35.5)               = 189.9
mass TiCl₄ = 1565.8 × 189.9 = 297 × 10³ g   = 297 kg

Keep unrounded values in your calculator until the last step.

The hydroxides: solubility and uses

Trend: the solubility of the hydroxides increases down the group.

Mg(OH)₂  sparingly soluble    (least soluble)
Ca(OH)₂  slightly soluble
Sr(OH)₂  more soluble
Ba(OH)₂  most soluble

Because more hydroxide dissolves further down the group, a saturated solution of Ba(OH)₂ contains a higher concentration of OH⁻ ions and so has a higher pH than a saturated solution of Mg(OH)₂.

Mg(OH)₂ in medicine. Magnesium hydroxide is used as an antacid to neutralise excess stomach acid (hydrochloric acid):

Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O

Because it is only sparingly soluble, the concentration of hydroxide ions it releases stays low.

Ca(OH)₂ in agriculture. Calcium hydroxide (slaked lime) is spread on acidic soil to raise the soil pH, which is needed for good crop growth:

Ca(OH)₂ + 2H⁺ → Ca²⁺ + 2H₂O

Removing SO₂ from flue gases

Burning fossil fuels that contain sulfur releases sulfur dioxide, an acidic gas. Calcium oxide and calcium carbonate are bases, so they react with SO₂ and remove it from flue gases:

CaO(s)   + SO₂(g) → CaSO₃(s)
CaCO₃(s) + SO₂(g) → CaSO₃(s) + CO₂(g)

Note that the calcium carbonate route releases carbon dioxide.

Worked example 3: how much base?

A power station must remove 1.00 tonne of SO₂. Calculate the mass of CaCO₃ needed, and the mass of CaO that would do the same job. (Aᵣ: Ca = 40.1, C = 12.0, O = 16.0, S = 32.1)

Mr(SO₂)   = 32.1 + 32.0 = 64.1
n(SO₂)    = 1.00 × 10⁶ g ÷ 64.1 = 1.560 × 10⁴ mol
Mr(CaCO₃) = 100.1   mass CaCO₃ = 1.560 × 10⁴ × 100.1 = 1.56 × 10⁶ g = 1.56 tonnes
Mr(CaO)   = 56.1    mass CaO   = 1.560 × 10⁴ × 56.1  = 8.75 × 10⁵ g = 0.875 tonnes

The sulfates: solubility, the sulfate test and BaSO₄ in medicine

Trend: the solubility of the sulfates decreases down the group, the opposite of the hydroxides.

MgSO₄  soluble        (most soluble)
CaSO₄  slightly soluble
SrSO₄  less soluble
BaSO₄  insoluble      (least soluble)

Testing for sulfate ions

Add dilute hydrochloric acid (or dilute nitric acid) to the sample, then add barium chloride solution. A white precipitate of barium sulfate shows that sulfate ions are present:

Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)

Why BaCl₂? Barium sulfate is insoluble, so if sulfate ions are present a precipitate forms even at low concentrations.

Why acidify? Carbonate ions would also give a white precipitate with barium ions (BaCO₃), and so would sulfite ions. The acid removes them first:

2H⁺(aq) + CO₃²⁻(aq) → CO₂(g) + H₂O(l)

Do not acidify with sulfuric acid: it adds sulfate ions, so you would always see a precipitate.

BaSO₄ in medicine

Barium sulfate is used in a “barium meal”. The patient swallows a suspension of BaSO₄, which is opaque to X-rays, so the outline of the digestive system shows up on an X-ray image. Soluble barium compounds are toxic, but barium sulfate is insoluble, so barium ions are not absorbed into the blood.

Required practical 4: identifying Group 2 cations

The specification lists test-tube reactions to identify Group 2 cations and the anions OH⁻, CO₃²⁻, SO₄²⁻ and the halide ions. For Group 2, the two solubility trends give you two tests. Use solutions of similar concentration so the comparison is fair.

Add to the Group 2 ion solution	Mg²⁺	Ca²⁺	Sr²⁺	Ba²⁺
sodium hydroxide solution	white precipitate	less precipitate	little	least or none
dilute sulfuric acid	no precipitate	slight	more	white precipitate

Read each row from the trend: hydroxides get more soluble down the group, so the precipitate gets smaller; sulfates get less soluble, so it gets larger.

Common errors

• Saying ionisation energy decreases “because there is more nuclear charge”. Nuclear charge increases; the deciding factors are distance and shielding.
• Writing that Mg reacts with steam to form Mg(OH)₂. With steam the product is MgO.
• Mixing up the two solubility trends. Hydroxides: more soluble down the group. Sulfates: less soluble.
• Describing Mg(OH)₂ as “insoluble”. The specification calls it sparingly soluble; BaSO₄ is the one it calls insoluble.
• Saying the acid in the sulfate test “reacts with the sulfate”. It removes carbonate (and sulfite) ions.
• Explaining the safety of a barium meal by saying “barium is not toxic”. Barium ions are toxic; BaSO₄ is safe because it is insoluble.
• Calling magnesium the oxidising agent in the titanium extraction. Magnesium is oxidised, so it is the reducing agent.

Where to go next

Condense this into the revision notes, then test it with the practice questions. For paper structure, see AQA A-Level Chemistry exam preparation.

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).