OCR A-Level Chemistry AS Breadth in chemistry (01), June 2018: Question 1

✅ Correct Answer

C: Na₂X

💡 Key Knowledge

• Count the total electrons or look at the outer shell: 3s²3p⁴ indicates 6 electrons in the outer principal energy level ($n=3$).
• Element X is Sulfur (S), which needs 2 electrons to achieve a stable noble gas octet, forming an X²⁻ ion.
• Sodium (Na) is in Group 1 and loses 1 electron to form a Na⁺ ion.

🧠 Exam Technique

Cross-multiply the charges or balance the ion numbers: you need two Na⁺ ions to balance the single 2⁻ charge of ion X, giving the formula Na₂X .

❌ Common Errors

Students frequently confuse the number of outer-shell electrons (6) with the ionic charge required to complete the octet, mistakenly thinking it forms a 6⁻ ion or leads to a 1:1 ratio like NaX .