What is the complete ground-state electron configuration of a neutral iron atom (atomic number Z = 26)?
Answer and explanation
Correct answer: B. 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶
- Step 1: Follow the Aufbau principle filling orbitals in order of increasing energy: 1s, 2s, 2p, 3s, 3p, 4s, 3d.
- Step 2: Distribute 26 electrons: [Ar] contains 18 electrons (1s² 2s² 2p⁶ 3s² 3p⁶), then 4s fills with 2 electrons, and the remaining 6 electrons enter 3d: [Ar] 4s² 3d⁶.
Answer
1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶.
Why not the others: 3d⁸ omits the filled 4s subshell; 4s¹ 3d⁷ violates Hund's/Aufbau ground state for iron; 4s² 4p⁶ represents krypton configurations.
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