JEE Main · 2012 · 29 Apr 2012 · Offline · Classification of Elements and Periodicity

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

(A) $Cl^{-}, Ca^{2 +}, K^{+}, S^{2 -}$
(B) $S^{2 -}, Cl^{-}, Ca^{2 +}, K^{+}$
(C) $Ca^{2 +}, K^{+}, Cl^{-}, S^{2 -}$ ✓ Correct
(D) $K^{+}, S^{2 -}, Ca^{2 +}, Cl^{-}$

Answer: (C)

Solution

The four species each carry

18

electrons, so for this isoelectronic set the radius shrinks as the nuclear charge rises, since more protons pull the same electron cloud inward. The nuclear charges increase through

S^{2 -} (16)

C l^{-} (17)

K^{+} (19)

and

C a^{2 +} (20)

, so the radius increases in the opposite order,

C a^{2 +} < K^{+} < C l^{-} < S^{2 -}

JEE Main · 2012 · 29 Apr 2012 · Offline · Single correct option

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