WAEC 2019 Chemistry · Question 50
The number of electrons in the 3d orbital of Cr is
Options
- A.2
- B.3
- C.4
- D.5
Answer: D. 5
Explanation written with AI and checked against the answer key
Analyze the question: We need the number of electrons in the 3d sublevel of chromium (atomic number 24). Concept: Normally, electrons fill orbitals in the order 1s, 2s, 2p, 3s, 3p, 4s, 3d... However, chromium is an exception. A half-filled d sub-shell (d⁵) is especially stable because of symmetry and exchange energy. So one electron from 4s moves into 3d. Solution: Expected (by the simple Aufbau order): 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁴ Actual (the stable configuration): 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹ 3d⁵ Check the total: 2 + 2 + 6 + 2 + 6 + 1 + 5 = 24 ✓ So the 3d orbitals contain 5 electrons, one in each of the five d orbitals. Answer: D. 5. Exam tip: Remember the two famous exceptions in the first row of the d-block: Cr (24) is [Ar] 4s¹ 3d⁵ and Cu (29) is [Ar] 4s¹ 3d¹⁰. Half-filled and fully filled d sub-shells are extra stable. WASSCE and JAMB love to test this!AI can make mistakes. Report anything that looks wrong.
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