Properties and Reactions of Group 2 Elements — AQA A-level Chemistry

SPEC 3.2.2 - GROUP 2, THE ALKALINE EARTH METALS

Group 2 Trends, Reactions, Solubility & Uses

Trends in properties, reactions with water, solubility patterns, and applications of Group 2 compounds

Introduction to Group 2

Definition

The alkaline earth metals are the elements in Group 2: beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra).

All Group 2 elements have two electrons in their outermost principal quantum shell (ns² configuration). They form ionic compounds by donating these two electrons to become M²⁺ ions, achieving a stable noble gas electron configuration.

When Group 2 metals react, they act as reducing agents - they donate electrons to other species and become oxidised themselves. Going down the group, the metals become more reactive.

Learning Objectives

Explain trends in atomic radius, first ionisation energy, and melting point

Describe reactions of Group 2 metals with water and oxygen

Compare solubilities of hydroxides and sulfates

Understand uses of Group 2 compounds

Perform the test for sulfate ions

Trend: Ionisation Energy

The first ionisation energy is the energy needed to remove the first outer electron. The second ionisation energy is the energy needed to remove the second outer electron. Both decrease going down Group 2, explaining why metals become more reactive.

Ionisation energy decreases down Group 2 due to three competing factors:

Nuclear charge - Increases down the group (more protons), increasing attraction for electrons

Atomic radius - Increases as more electron shells are added, moving electrons further from nucleus

Electron shielding - Increases as more inner electron shells reduce nuclear attraction

The atomic radius and shielding effects are greater than the nuclear charge effect, leading to an overall decrease in ionisation energy.

Exam Tip

Always link: more shells → greater distance → more shielding → weaker attraction → less energy needed. This chain of reasoning is essential for full marks.

Trend: Atomic Radius

The Trend

Atomic radius increases going down Group 2 as each element has one more principal quantum shell than the element above.

Atomic radius increases down Group 2 because:

More inner electron shells are added as you move down the group

This results in increased shielding of outer electrons from the nucleus

The outermost electrons are found in higher energy levels and are on average further from the nucleus

With increasing shielding and increasing distance, the electrostatic attraction between nucleus and outer electrons decreases

ElementAtomic Radius (nm)1st I.E. (kJ mol⁻¹)Configuration

Mg0.160738[Ne] 3s²

Ca0.197590[Ar] 4s²

Sr0.215550[Kr] 5s²

Ba0.222503[Xe] 6s²

Worked Example01

Explain why the first ionisation energy of barium is lower than that of magnesium. [3 marks]

1

Barium has more electron shells than magnesium (6 vs 3), so the outer electron is further from the nucleus.

2

There is greater shielding of the outer electron by inner electrons in barium.

3

Therefore, weaker electrostatic attraction between nucleus and outer electron, so less energy required to remove it.

Key words: more shells / further / greater shielding / weaker attraction / less energy

Trend: Melting Point

Group 2 elements have metallic bonding: M²⁺ ions in a crystalline lattice surrounded by a sea of delocalised electrons.

Melting points generally decrease down the group because:

The ionic radius of the metal ion increases

Delocalised electrons get further away from the positive nuclei

The attractive force has to act over a much greater distance

Weaker electrostatic attraction between delocalised electrons and positive nuclei

Less energy needed to break the metallic bonds

Magnesium Anomaly

Magnesium has a lower melting point than expected (lower than Ca). This is because Mg forms an unusually stable crystal structure. Focus on the overall downward trend from Ca → Ba.

Reactions with Oxygen & Water

Reaction with Oxygen

2M(s) + O₂(g) → 2MO(s)

Flame colours:

Mg: Bright white

Ca: Red

Sr: Red

Ba: Green

Sr and Ba also form peroxides (MO₂). Ba is so reactive it must be stored in oil.

Reaction with Water

M(s) + 2H₂O(l) → M(OH)₂(s) + H₂(g)

Reactivity increases down the group (lower IE → easier to form M²⁺).

Mg reacts very slowly with cold water but vigorously with steam:

Mg + H₂O(g) → MgO + H₂

ElementReaction with WaterObservations

BeNo reaction—

MgVery slow; vigorous with steamFew bubbles; Mg(OH)₂ layer; pH 9-10

CaSteady reaction rateBubbles of H₂; milky Ca(OH)₂

SrReacts fairly quicklyRapid bubbling

BaRapid reactionVigorous H₂ evolution

Mg with Steam

Mg reacts much faster with steam as it provides extra energy. Burns with a bright white flame to form hydrogen and magnesium oxide (a white powder).

Redox

These are redox reactions. The metal is oxidised from oxidation state 0 to +2 (loses electrons). Example: Mg → Mg²⁺ + 2e⁻

Reactions with Acids

With dilute HCl: Bubbles of H₂ given off much faster down the group. Ba reacts so vigorously it must be stored in oil.

With dilute H₂SO₄: Ca, Sr, and Ba form insoluble sulfate layers on the metal surface which stop the reaction. This doesn't happen with HCl or HNO₃ as they form soluble salts.

Worked Example02

Write an equation for the reaction of calcium with water and explain why calcium reacts more vigorously than magnesium. [4 marks]

1

Equation: Ca + 2H₂O → Ca(OH)₂ + H₂ [1 mark]

2

Calcium has a lower first ionisation energy than magnesium [1 mark]

3

Because Ca has more electron shells / outer electrons further from nucleus / more shielding [1 mark]

4

Therefore easier to form Ca²⁺ ions / easier to lose electrons, so reaction is faster [1 mark]

Solubility of Hydroxides

The Trend

The solubility of Group 2 hydroxides increases down the group.

Mg(OH)₂ is sparingly soluble → Ba(OH)₂ is soluble

Note

All Group 2 oxides are basic, except for BeO which is amphoteric (can act as both an acid and a base).

When Group 2 oxides react with water, they form metal hydroxides:

MO(s) + H₂O(l) → M(OH)₂(aq)

This releases hydroxide ions (OH⁻), making the solutions alkaline. The pH of the resulting solution increases down the group as hydroxide solubility increases. MgO is an exception - it reacts slowly and has low solubility.

ElementHydroxide SolubilitySulfate Solubility

MgInsoluble (sparingly)Soluble

CaSparingly solubleSparingly soluble

SrSolubleInsoluble

BaSolubleInsoluble

Solubility of Sulfates

The Trend

The solubility of Group 2 sulfates decreases down the group.

MgSO₄ is soluble → BaSO₄ is insoluble

This is the opposite trend to hydroxides! Magnesium sulfate (Epsom salts) is highly soluble, whereas barium sulfate is essentially insoluble.

Memory Aid

"HI, SD" - Hydroxides Increase, Sulfates Decrease

BaSO₄ in Medicine

Barium meal: Used for X-ray imaging of digestive system. Although Ba²⁺ is toxic, BaSO₄ is safe because it's so insoluble that Ba²⁺ ions cannot enter the bloodstream.

Worked Example03

Predict the relative solubilities of strontium hydroxide and strontium sulfate. Explain your reasoning. [3 marks]

1

Sr(OH)₂: Fairly soluble - more soluble than Mg(OH)₂ but less than Ba(OH)₂ [1 mark]

2

Because hydroxide solubility increases down Group 2 [1 mark]

3

SrSO₄: Sparingly soluble / insoluble - less soluble than MgSO₄ but more than BaSO₄. Sulfate solubility decreases down Group 2 [1 mark]

Uses of Group 2 Hydroxides

Mg(OH)₂ - Antacid

"Milk of magnesia" neutralises excess stomach acid. Safe because only partially soluble, making the solution only slightly alkaline (pH ≈ 10) due to low OH⁻ concentration.

Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O

Ca(OH)₂ - Agriculture

Slaked lime neutralises acidic soils ("liming"). Optimum pH for crops is 6-6.5. Ca(OH)₂ in solution is called limewater.

Ca(OH)₂ + 2H⁺ → Ca²⁺ + 2H₂O

Ionic Equation

All neutralisation reactions can be summarised by:H⁺(aq) + OH⁻(aq) → H₂O(l)

Calcium Compounds

Limestone = impure calcium carbonate (CaCO₃)

Quicklime = calcium oxide (CaO), from thermal decomposition of CaCO₃

Slaked lime = calcium hydroxide (Ca(OH)₂), from adding water to quicklime

Industrial Uses

Flue Gas Desulfurisation

Power stations use CaO or CaCO₃ slurry to remove SO₂ (sulfur scrubbing), preventing acid rain.

CaO + SO₂ → CaSO₃

CaCO₃ + SO₂ → CaSO₃ + CO₂

Titanium Extraction (Kroll)

Step 1: Convert TiO₂ to TiCl₄

TiO₂ + 2C + 2Cl₂ → TiCl₄ + 2CO

Step 2: Purify TiCl₄ by fractional distillation, then reduce with Mg (displacement)

TiCl₄ + 2Mg → Ti + 2MgCl₂

Why Magnesium?

Carbon cannot be used because titanium carbide (TiC) forms instead of titanium metal. Magnesium is a stronger reducing agent than titanium - it is more reactive and readily donates electrons to reduce Ti⁴⁺ to Ti. Sodium can also be used as an alternative.

Testing for Sulfate Ions

The Test

Add acidified barium chloride solution (BaCl₂ in dilute HCl) to the test solution.

OR: Add dilute HNO₃ followed by Ba(NO₃)₂ solution.

Positive result: White precipitate of barium sulfate.Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)

Why Acidify?

The acid removes carbonate and sulfite ions which would also give white precipitates with Ba²⁺, ensuring the test is specific to sulfate:

CO₃²⁻ + 2H⁺ → H₂O + CO₂

SO₃²⁻ + 2H⁺ → H₂O + SO₂

BaSO₄ is unaffected - sulfate ions don't react with H⁺.

Exam Warning

You MUST state that barium chloride is acidified. Just "add barium chloride solution" loses marks.

Worked Example04

An aqueous solution of a Group 2 metal chloride, XCl₂, forms a white precipitate when dilute NaOH is added. A separate sample does NOT form a precipitate when dilute Na₂SO₄ is added. Identify metal X and write equations for any reactions. [4 marks]

1

White precipitate with NaOH means the hydroxide is insoluble/sparingly soluble [1 mark]

2

No precipitate with Na₂SO₄ means the sulfate is soluble [1 mark]

3

X = Magnesium (Mg(OH)₂ is insoluble, MgSO₄ is soluble) [1 mark]

4

MgCl₂(aq) + 2NaOH(aq) → Mg(OH)₂(s) + 2NaCl(aq) [1 mark]

Practice

Question 1: When calcium reacts with dilute sulfuric acid, the reaction quickly slows down and stops. Explain why this happens and suggest how you could use a different acid to react calcium completely. [3 marks]

1

Calcium sulfate (CaSO₄) is sparingly soluble / insoluble

2

It forms a layer on the surface of the calcium metal, preventing further reaction / acting as a barrier

3

Use dilute HCl or HNO₃ instead, as these form soluble salts (CaCl₂ or Ca(NO₃)₂)

Practice

Question 2: Barium sulfate is used in medicine despite barium ions being toxic. Explain why barium sulfate is safe for X-ray imaging of the digestive system. [2 marks]

1

Barium sulfate is insoluble in water

2

Ba²⁺ ions cannot dissolve and enter bloodstream / be absorbed, so no toxic effect

Practice

Question 3: Explain why magnesium hydroxide is preferred over calcium hydroxide as an antacid for treating excess stomach acid. [3 marks]

1

Mg(OH)₂ is sparingly soluble / has low solubility in water

2

This means it produces a low concentration of OH⁻ ions / weakly alkaline (pH ~10)

3

Ca(OH)₂ is more soluble so would be too strongly alkaline / could damage stomach lining / cause irritation

Summary

Key Trends (Mg → Ba)

PropertyTrendKey Reason

Atomic radiusIncreasesMore shells, more shielding

1st & 2nd IEDecreaseWeaker attraction to outer e⁻

Melting pointDecreases (generally)Lower charge density

ReactivityIncreasesEasier to form M²⁺

Hydroxide solubilityIncreases"HI" - Hydroxides Increase

Sulfate solubilityDecreases"SD" - Sulfates Decrease

Key Exam Skills

Explain trends: shells → distance/shielding → attraction → property

Water reactions: M + 2H₂O → M(OH)₂ + H₂ (Mg + steam → MgO + H₂)

Sulfate test: acidified BaCl₂ (or HNO₃ + Ba(NO₃)₂) → white BaSO₄

Uses: Mg(OH)₂ antacid, Ca(OH)₂ agriculture, Mg for Ti extraction