WJEC A-Level Chemistry AS Unit 1, June 2025: Question 4

✅ Correct Answer

XCl

📐 Step-by-Step Deduction

1. Locate the largest gap: The ionisation energy increases from 402 to 2650 kJ mol⁻¹ (a huge increase of 2248 kJ mol⁻¹).
2. Deduce valence electrons: Only 1 electron is easily removed before a significantly higher energy is required.
3. Identify the Group: Element X is in Group 1 (alkali metals).
4. Determine ion charges: Group 1 forms X⁺ ions; chlorine forms Cl⁻ ions.
5. Combine 1:1: X⁺ + Cl⁻ → XCl .

💡 Key Knowledge

• Why the big jump occurs: The 2nd electron is removed from an inner quantum shell (closer to the nucleus) where it experiences significantly less electron shielding and a much stronger effective nuclear charge.
• Gradual increases: Between the 2nd, 3rd, and 4th ionisation energies, values increase steadily because electrons are being removed from an increasingly positive ion, but from the same shell.

🧠 Exam Technique

• Use the symbol provided: The question asks for the chloride of X. Always use X as the symbol—do not guess and write actual element symbols like NaCl or KCl .
• No ionic charges in compounds: Write formulas as neutral compounds ( XCl ), never with separate charges left in (e.g., avoid writing X⁺Cl⁻ ).
• Watch capitalisation: Elements must have correct case sensitivity ( XCl , not XCL or xcl ).

❌ Common Errors

• Writing XCl₂: Mistakenly counting two numbers before noticing the jump, or misidentifying X as Group 2.
• Writing actual elements: Writing NaCl or KCl instead of using the placeholder letter X given in the question stem.
• Confusing shell removal: Thinking the jump means the element has 2 valence electrons instead of 1. Remember: the jump represents moving into an inner core shell!