OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2025: Question 14

Periodic Trends & Properties of Group 2: Barium

📋 What this question tests

This multiple-choice question assesses your core understanding of Group 2 chemistry and periodic trends: electronic configurations (s-block classification), the trend in reactivity of alkaline earth metals down the group with water, and the factors controlling first ionisation energy down a group.

Question 14 Analysis

Evaluating Statements 1, 2, and 3

Identifying valid statements regarding Barium (Ba)

✅ Correct Answer: B

Only 1 and 2 are correct.

Statement 1 is TRUE: Barium is in Group 2 (Period 6), so its outer electron configuration is [Xe] 6s². The outer shell electrons are in an s-orbital.
Statement 2 is TRUE: Reactivity with water increases down Group 2. Barium reacts vigorously with cold water, whereas magnesium reacts extremely slowly.
Statement 3 is FALSE: First ionisation energy decreases down Group 2. Barium has a lower first ionisation energy than magnesium.

Mark Scheme Breakdown

Answer: B [1 Mark]

OCR Multiple Choice format: 1 mark awarded for selecting option B. No partial marks are available for individual statements.

Detailed Statement Breakdown

💡 Key Knowledge: Trends Down Group 2

Position in Periodic Table: All Group 2 elements have an outer shell configuration of ns² , making them part of the s-block.
Reaction with Water:
M(s) + 2H₂O(l) → M(OH)₂(aq) + H₂(g)
Reactivity increases down the group as outer electrons are lost more readily.
First Ionisation Energy:
M(g) → M⁺(g) + e⁻
Decreases down the group due to increased atomic radius and greater electron shielding outweighing the increase in nuclear charge.

🧠 Exam Technique: "1, 2, and 3" MCQs

Eliminate systematically: Evaluate statement 3 first if it is straightforward. Since statement 3 is clearly false (IE decreases down a group), you can immediately eliminate A and C.
Decide between the remaining options: You are left with B (1 and 2) or D (1 only). Now evaluate statement 2. Knowing barium is more reactive than magnesium confirms statement 2 is true, leaving B as the definite answer.

Common Pitfalls & Misconceptions

❌ Common Errors to Avoid

Nuclear Charge Fallacy: Students often assume that because barium has a much higher nuclear charge (56 protons vs 12 protons for magnesium), its first ionisation energy must be higher. Remember: shielding and atomic radius outweigh nuclear charge down any group.
Confusing s-block and d-block: Barium is in Period 6 where the 5d subshell begins filling in lanthanides/transition elements, but barium's outermost valence electrons are strictly in the 6s orbital.
Confusing Group 2 reactivity with Group 7: Group 2 reactivity increases down the group (easier to lose electrons), whereas Group 7 (halogens) reactivity decreases down the group (harder to gain electrons).

📐 Quick Reference: Mg vs Ba Comparison

Outer electron shell: Mg is 3s² ; Ba is 6s²
Reaction with cold water:
• Mg: Extremely slow reaction producing a cloudy suspension of Mg(OH)₂.
• Ba: Rapid effervescence producing an alkaline solution of Ba(OH)₂.
First Ionisation Energy:
• Mg: 738 kJ mol⁻¹
• Ba: 503 kJ mol⁻¹ (significantly lower!)