OCR A-Level Chemistry AS Breadth in chemistry (01), June 2022: Question 4

✅ Correct Answer

C (7)

Sulfur has an atomic number of 16. Its electron configuration is 1s² 2s² 2p⁶ 3s² 3p⁴ . Counting the fully filled (paired) orbitals gives: 1s (1), 2s (1), 2pₓ (1), 2pᵧ (1), 2pᵂ (1), 3s (1), and one of the 3p orbitals (1), totaling 7 paired orbitals.

💡 Key Knowledge

• Atomic Number: Sulfur (S) has 16 electrons.
• Subshell Capacity: s-subshell holds max 2 electrons (1 orbital); p-subshell holds max 6 electrons (3 orbitals).
• Hund's Rule: Electrons fill degenerate orbitals singly before pairing up. Therefore, in the 3p⁴ subshell, one 3p orbital contains a pair, and the other two contain single electrons.

🧠 Exam Technique

Don't guess! Write out the full subshell electron configuration first. Then, draw or visualize the "boxes" (orbitals) for each subshell to explicitly count which ones contain two electrons versus one.

❌ Common Errors

• Confusing electrons with orbitals: Students often count the total number of paired electrons (which would be 14) rather than the number of orbitals holding pairs.
• Miscounting core electrons: Forgetting that inner shells ( 1s , 2s , 2p ) are completely full and each contribute fully to the paired orbital count.