Chemistry Atomic Structure PMDC 2020
PMDC Verified Question 73 of 95
Which pair has 1 electron in its outer most '\(s\)' orbital?
A
Li & Fe
B
Na & Cr
C
K & Mn
D
H & He
Tap any option to test your recall and reveal the step-by-step Propolis autopsy.

Propolis Cognitive Error Autopsy

Official Correct Choice:
Option B: Na & Cr
Concept: Elements in Group 1 (Alkali metals) naturally have \( ns^1 \) configurations. Certain transition metals exhibit anomalous configurations resulting in an \( ns^1 \) outer shell.

Formula: None required.

Solution:
  • Sodium (Na, \(Z=11\)): \( [Ne] 3s^1 \) (1 electron in outermost \(s\)).
  • Chromium (Cr, \(Z=24\)): \( [Ar] 4s^1 3d^5 \) (anomalous config to achieve half-filled stability, leaving 1 electron in the outermost \(s\)).
  • Therefore, the pair Na & Cr both fit the condition.


Why other options are incorrect: Iron (Fe) is \( 4s^2 \). Manganese (Mn) is \( 4s^2 \). Helium (He) is \( 1s^2 \).

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.

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