WAEC 2023 Chemistry · Question 18
How many unpaired electrons are present in ?
Options
- A.4
- B.2
- C.3
- D.5
Answer: D. 5
Explanation written with AI and checked against the answer key
Step 1: Analyze the question We need the number of unpaired electrons in the Fe³⁺ ion. Iron has atomic number 26, so a neutral atom has 26 electrons. Step 2: Concept When transition metals form positive ions, they lose the 4s electrons first, then 3d electrons. Step 3: Solution Neutral Fe (26 electrons): 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶ Fe³⁺ (23 electrons): remove 2 electrons from 4s and 1 from 3d: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵ Now place the 5 electrons in the five 3d orbitals using Hund's rule (one electron in each orbital before pairing): 3d: ↑ ↑ ↑ ↑ ↑ All 5 electrons are unpaired. Answer: 5 unpaired electrons, which is option D. **Tip: A half-filled d subshell (d⁵) is especially stable, which is why Fe³⁺ is a stable ion. Remember, for transition metal ions: "4s leaves first." Also, Fe²⁺ (3d⁶) has 4 unpaired electrons, a common trap that leads to option A.AI can make mistakes. Report anything that looks wrong.
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