OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), November 2020: Question 14

1 mark · Hard difficulty · Multiple Choice

Identify the correct statements concerning Group 2 elements regarding ionisation energy, electron configuration of ions, and reactions with water.

Practise this question

Question

Multiple choice question 14 asking which statements for Group 2 elements are correct. Statement 1: The 2nd ionisation energy of magnesium is greater than the 2nd ionisation energy of calcium. Statement 2: A strontium ion, Sr2+, contains a total of 6 electrons in s orbitals. Statement 3: The equation for the reaction of barium with water is: 2Ba + 2H2O -> 2BaOH + H2. Options A (1, 2 and 3), B (Only 1 and 2), C (Only 2 and 3), and D (Only 1) are listed, with an answer box at the bottom.
Question text

14 Which statement(s) for Group 2 elements is/are correct?

1 The 2nd ionisation energy of magnesium is greater than the 2nd ionisation energy of

calcium.

2 A strontium ion, Sr2+, contains a total of 6 electrons in s orbitals.

3 The equation for the reaction of barium with water is:

2Ba + 2H2O 2BaOH + H2.

A 1, 2 and 3

B Only 1 and 2

C Only 2 and 3

D Only 1

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme indicating the correct answer is D.

14 D 1 1.2 ALLOW 1 in the answer box

How to answer it

Group 2 Elements Properties and Reactions

OCR A-Level Chemistry — Multiple Choice Question

What this question tests

This multi-statement question tests your deep understanding of Group 2 trends, specifically successive ionisation energies down a group, electronic configurations of ions, and equations for chemical reactions between Group 2 metals and water.

Question Breakdown & Analysis

✅ Correct Answer: D (Only 1)

Statement 1 is the only correct statement among the three options provided.

🧠 Exam Technique

Evaluate each numbered statement independently as true or false. Cross out options that contain incorrect statements to rapidly narrow down to the correct multiple-choice letter.

Detailed Statement Analysis

Statement 1: The 2nd ionisation energy of magnesium is greater than the 2nd ionisation energy of calcium. (TRUE)

  • Reasoning: Removing a 2nd electron from magnesium involves an ion with a smaller ionic radius and less shielding than calcium (Mg⁺ vs Ca⁺).
  • Furthermore, the 2nd electron removed from magnesium is from the 2s subshell (closer to the nucleus), whereas for calcium it is from the 4s subshell. This leads to a much stronger nuclear attraction for magnesium, making its 2nd ionisation energy higher.

Statement 2: A strontium ion, Sr²⁺, contains a total of 6 electrons in s orbitals. (FALSE)

  • Full electron configuration of Strontium (Sr): 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s²
  • Configuration of Sr²⁺ ion: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ (losing the two 5s electrons).
  • Counting s-electrons: Look at all the s orbitals (1s², 2s², 3s², 4s²). There are 4 energy levels with full s orbitals, giving 2 + 2 + 2 + 2 = 8 electrons in s orbitals, not 6.

Statement 3: The equation for the reaction of barium with water is 2Ba + 2H₂O → 2BaOH + H₂ (FALSE)

  • The Trap: The formula given for barium hydroxide is incorrect ( BaOH instead of Ba(OH)₂ ). Barium is in Group 2, so it forms a 2+ ion, requiring two hydroxide ions ( OH⁻ ) for electrical neutrality.
  • Correct balanced equation: Ba + 2H₂O → Ba(OH)₂ + H₂
Mark Allocation: [1 mark] awarded for selecting option D.

Topics

Module 3: Periodic table and energy · Module 2: Foundations in chemistry · 3.1 The periodic table · 2.2 Electrons, bonding and structure

Question and mark scheme from the OCR A-Level Chemistry examination, Periodic table, elements and physical chemistry (01), November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.