alleryBy the AUFBAU principle, the 3d subshell starts filling:
AAfter 4s is filled (3d > 4s in energy by n+l rule)
BBefore 4s starts filling (3d below 4s, incorrect)
CSimultaneously with 4s in all transition elements
DOnly after 5s is filled in transition elements
Answer & Solution
Correct answer: A. After 4s is filled (3d > 4s in energy by n+l rule)
**Aufbau (n+l) rule:** 4s has n+l = 4, while 3d has n+l = 5. Lower (n+l) fills first, so 4s fills before 3d.
Then 3d fills (start at Sc, Z=21).
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