Chemistry S & P Block Elements MDCAT 2017
PMDC Verified Question 85 of 104
Ionic radius along the period decreases due to:
A
Addition of a new shell
B
High Ionization energy
C
Increase in nuclear charge
D
Decrease in nuclear charge
Tap any option to test your recall and reveal the step-by-step Propolis autopsy.

Propolis Cognitive Error Autopsy

Official Correct Choice:
Option C: Increase in nuclear charge
Concept:

For isoelectronic series (ions with the same electron configuration) across a period, the dominant factor determining size is the positive pull of the nucleus.

Solution:

  • As we move along a period (e.g., forming isoelectronic cations like \( \text{Na}^+, \text{Mg}^{2+}, \text{Al}^{3+} \)), the electrons are present in the exact same valence shell, so shielding remains constant.
  • However, the atomic number (number of protons) increases, leading to an increase in effective nuclear charge.
  • This stronger nuclear pull draws the electron cloud tighter, thereby decreasing the ionic radius.


Why other options are incorrect:

  • Option A: Shells are not added across a period.
  • Option B: High IE is a consequence of increased nuclear pull, not the direct physical cause of the radius contraction.
  • Option D: Nuclear charge increases, it does not decrease.

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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