OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2018: Question 6

✅ Correct Answer: C (X₃Y₂)

Option C is the correct chemical formula. Element X forms a 2+ cation, and element Y forms a 3- anion. Balancing the charges requires three X²⁺ ions and two Y³⁻ ions to give a neutral compound: 3(+2) + 2(-3) = 0 .

💡 Key Knowledge

• Group 2 elements: Lose 2 electrons to form stable 2+ cations (e.g., Mg²⁺, Ca²⁺).
• Group 15 elements: Gain 3 electrons to achieve a stable octet, forming 3- anions (e.g., N³⁻, P³⁻).
• Ionic lattices: Overall, the compound must be electrically neutral, meaning total positive charge must equal total negative charge.

🧠 Exam Technique

When given group numbers instead of element symbols, quickly jot down the typical ion charges in the margin. Use the "swap and drop" method if unsure, but always double-check that the total charges balance to zero.

❌ Common Errors

Students frequently confuse group numbers with valencies or accidentally invert the subscripts (e.g., choosing option B: X₂Y₃ by swapping the charges incorrectly or misattributing the 2+ and 3- ions to the wrong elements).