Verified Solution • 2025–26 Session moderate

QJEEM12Ch1e25: JEE Main 2012 Chemistry - Class 11 JEEM12Ch1e25 Solution: The increasing order of the ionic radii of the given…

The increasing order of the ionic radii of the given isoelectronic species is :

Multiple Choice Options

  • A Cl–, Ca2+ , K+ , S2–
  • B S2–, Cl–, Ca2+ , K+
  • C Ca2+ , K+ , Cl– , S2–
  • D K+, S2–, Ca2+, Cl–

Step-by-Step Verified Solution

Iso-electronic ions have same number of electrons. So, for iso-electronic ions, number of electrons = constant.
∴ σ(Slaten's Constant) = Constant. As σ depends on number of electrons. If a element's number of electrons increases then that element's σ increases.

Also we know, Zeffective = Z - σ

As for iso-electronic ions, σ(Slaten's Constant) = Constant. So Zeffective depend on only value of Z. If Z of an ion increases then Zeffective also increases and if Z of an ion decreases then Zeffective also decreases. And when Zeffective increases then nuclear attraction towards outermost electrons increase and size of ion decreases. Similarly when Zeffective decrease then nuclear attraction towards outermost electrons decreases and size of ion increases.

∴ Size ∝ 1ZefforZ

Ca2+ K+ Cl- S2-
Z 20 19 17 16

∴ Ionic radius order

Ca2+ < K+ < Cl– < S2–

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