alleryThe Aufbau principle dictates that orbitals fill in order of increasing
A$n \cdot l$
B$n$ only
C$n + l$ (with lower $n$ first when $n+l$ ties)
D$l$ only
Answer & Solution
Correct answer: C. $n + l$ (with lower $n$ first when $n+l$ ties)
$n + l$ rule (Madelung): smaller $n+l$ first
ties broken by smaller $n$.
**So 4s ($n+l=4$) fills before 3d ($n+l=5$) — explains the d-block 'late' start.**
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