OCR A-Level Chemistry AS Breadth in chemistry (01), June 2022: Question 6

✅ Correct Answer

B ( XCl₂ )

Element X has 2 valence electrons, meaning it belongs to Group 2 (magnesium, Mg). When reacting with chlorine (Group 17), it forms an ionic compound with a 2+ metal ion and 1- chloride ions, resulting in the formula XCl₂ .

💡 Key Knowledge

• Successive Ionisation Energies: Values increase as each electron is removed due to a decreasing shielding effect and increasing net positive charge.
• Finding Shell/Group: A huge, sharp jump in ionisation energy indicates that an electron is being removed from a new, inner principal quantum shell closer to the nucleus.
• Group 2 Behaviour: Element X shows a massive jump between the 2nd and 3rd ionisation energies (1451 to 7733 kJ mol⁻¹), proving there are 2 electrons in the outer shell.

🧠 Exam Technique

• Scan the data horizontally from left to right looking for the largest proportional difference between consecutive values.
• Count how many ionisation energies come before the huge jump. That number equals the group number for main-group elements.
• Link the identified group number directly to its typical ionic charge (+2 for Group 2) to deduce the formula with chlorine (-1).

❌ Common Errors

• Counting the wrong side: Students sometimes count the number of ionisation energies after the jump instead of before it.
• Confusing Period and Group: The question states it is a Period 3 element (so it is Mg), but answering requires knowing its Group number (Group 2), not its period.
• Incorrect valency translation: Mismatching charges when writing the ionic formula, leading to incorrect guesses like XCl or XCl₃ .