We define the location of every electron using three rules.
Electrons fill the lowest energy orbital available first.
Order: 1s 2s 2p 3s 3p 4s 3d 4p...
Note: 4s fills before 3d!
Max 2 electrons per orbital. Must have opposite spin.
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Orbitals of equal energy (degenerate) fill singly first to minimize repulsion.
THE EXCEPTIONS (Exam Favorite)
Half-filled (\(d^5\)) and fully-filled (\(d^{10}\)) sublevels offer extra stability.
[Ar] 4s² 3d⁴
[Ar] 4s¹ 3d⁵
[Ar] 4s² 3d⁹
[Ar] 4s¹ 3d¹⁰
Writing Configurations for Ions
Problem: Write the electron configuration for the Iron(III) ion (\(Fe^{3+}\)). (Z=26).
1. Write Atom Config: Fe (26e⁻)
1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶
2. Remove Electrons:
RULE: Remove from the highest 'n' level first (4s), then 3d.
Remove 3 electrons: 2 from 4s, 1 from 3d.
3. Final Answer:
1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵
(Notice the stable half-filled d-subshell!)
⚠️ Examiner Tip / Common Mistake
When writing configurations for transition metal ions, always remove electrons from the 4s orbital BEFORE the 3d orbital. For example, a neutral Iron (Fe) atom is [Ar] 4s² 3d⁶, but an Fe²⁺ ion is [Ar] 3d⁶ (the 4s electrons are lost first!).
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