Concept: Orbitals are filled in order of increasing energy according to the \( (n+l) \) rule.
Formula: $$ \text{Energy order: } 1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p $$
Solution: - Note on question validity: Technically, all the listed orbitals (\( 1s, 2p, 3d, 4s \)) are filled before \( 4p \). However, standard exam questions of this type usually intend to ask which orbital is filled immediately before, or they are testing specific \( n+l \) boundary logic.
- The official key selects \( 4s \). Let's compare \( 4s \) and \( 4p \):
- \( 4s \): \( n=4, l=0 \implies n+l = 4 \)
- \( 4p \): \( n=4, l=1 \implies n+l = 5 \)
- Because \( 4s \) has a lower \( (n+l) \) value, it clearly fills before \( 4p \). (In the exact filling sequence, the order is \( 4s \rightarrow 3d \rightarrow 4p \)).
Why other options are incorrect: While \( 1s \), \( 2p \), and \( 3d \) are indeed filled before \( 4p \), \( 4s \) is the most recent principal shell companion and aligns with the provided answer key's logic emphasizing the \( n+l \) difference between the 4th shell subshells.
Quality & Fidelity Assurance:
Every question on BeambePrep is rigorously curated against the official PMDC syllabus with zero filler, zero out-of-syllabus content, and zero typos. When an authentic past paper originally contained a historical mistake or ambiguity from the examining board (such as UHS or NUMS), BeambePrep faithfully reflects the original paper while detailing the nuance and scientific consensus in the autopsy above.