OCR A-Level Chemistry AS Breadth in chemistry (01), June 2018: Question 9

✅ Correct Answer

A: YCl

Element Y is in Group 1 because the huge jump in ionisation energy occurs between the 1st and 2nd values. Group 1 elements form ions with a +1 charge, combining with chloride ions (Cl⁻) in a 1:1 ratio.

💡 Key Knowledge

• Successive Ionisation Energies: Removing each subsequent electron requires more energy because the remaining electrons are pulled closer to the increasingly positive nucleus.
• Identifying Groups: A massive, abrupt increase in ionisation energy indicates that an electron is being removed from a new, inner shell closer to the nucleus.
• Group 1 Clues: If the 1st IE is relatively low and the 2nd IE shows a massive jump, the element has 1 valence electron and belongs to Group 1 (e.g., Sodium, Potassium).

🧠 Exam Technique

• Scan the data row from left to right and look for the biggest jump in magnitude.
• Here, values go: 496 → 4563 (approx. a 9-fold increase!). Subsequent increases (4563 to 6913) are steady shell-filling increases.
• The jump occurs after the 1st electron is removed, meaning the 1st electron was the only valence electron in the outer shell. Therefore, element Y is in Group 1.
• Group 1 elements form Y⁺ ions, reacting with Cl⁻ to make YCl .

❌ Common Errors

• Miscounting the jump: Students sometimes confuse a steady increase across a shell with a shell change, or misinterpret which electron number corresponds to the inner shell breach.
• Defaulting to Group 2: Seeing multiple numbers makes students guess YCl₂ out of habit. Always check where the largest multiplicative leap occurs.
• Formula writing slip: Even when correctly identifying Group 1, students occasionally misapply valency rules for chlorides.