alleryThe Aufbau principle states that electrons fill atomic orbitals
AIn order of increasing energy
BIn any random order
CBy filling the last shell first
DBy pairing before filling
Answer & Solution
Correct answer: A. In order of increasing energy
1. Aufbau (German for "building up") principle guides electron configuration.
2. Electrons fill orbitals in order of increasing energy.
3. The (n + l) rule: fill lower (n + l) first; if equal, lower n first.
4. This gives sequence 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, …
_Source: syllabus Chemistry, Ch 2 "Structure of Atom", §2.6_
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