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.
ā ļø THE EXCEPTIONS (Exam Favorite)
Half-filled (\(d^5\)) and fully-filled (\(d^{10}\)) sublevels offer extra stability.
[Ar] 4s^2 3d^4
[Ar] 4s^1 3d^5
[Ar] 4s^2 3d^9
[Ar] 4s^1 3d^{10}
Writing Configurations for Ions
Paper 2 Style1. Write Atom Config: Fe (26eā»)
1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^6
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:
1s^2 2s^2 2p^6 3s^2 3p^6 3d^5
(Notice the stable half-filled d-subshell!)