Chemistry S & P Block Elements SET 2019
PMDC Verified Question 71 of 104
In the \( 3^{\text{rd}} \) period of the periodic table which of following property is decreasing consistently?
A
Electrical conductivity
B
Ionization energy
C
Melting point
D
Atomic radius
Tap any option to test your recall and reveal the step-by-step Propolis autopsy.

Propolis Cognitive Error Autopsy

Official Correct Choice:
Option D: Atomic radius
Concept:

Periodic properties follow specific trends based on electron shielding and effective nuclear charge.

Solution:

  • Across the 3rd period (from Sodium to Argon), electrons are added to the exact same 3rd principal shell, meaning the shielding effect remains roughly constant.
  • However, the nucleus gains one proton with each consecutive element, steadily increasing the effective nuclear charge \( (Z_{eff}) \).
  • This increasing nuclear magnet pulls the surrounding electron cloud progressively tighter.
  • As a result, the atomic radius decreases consistently across the period.


Why other options are incorrect:

  • Option A & C: Conductivity and melting points fluctuate (e.g., they increase initially and then drop sharply).
  • Option B: Ionization energy generally increases, not decreases.

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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