JEE Advanced · ChemistryHard

$Consider the following species: O^{2 -}, F^{-}, Na^{+}, Mg^{2 +} . Which of the following statements correctly describes the trend in their ionic radii and hydration energy?$

A. $Ionic radius increases from O^{2 -} to Mg^{2 +} and hydration energy decreases.$
B. $Ionic radius decreases from O^{2 -} to Mg^{2 +} and hydration energy increases.$
C. $Ionic radius decreases from O^{2 -} to Mg^{2 +} and hydration energy decreases.$
D. $Ionic radius increases from O^{2 -} to Mg^{2 +} and hydration energy increases.$

Show correct answer & step-by-step solution

Correct answer: B — $Ionic radius decreases from O^{2 -} to Mg^{2 +} and hydration energy increases.$

Solution

$The species O^{2 -}, F^{-}, Na^{+}, and Mg^{2 +} are isoelectronic with 10 electrons.$
$For isoelectronic species, ionic radius decreases as the effective nuclear charge increases from O^{2 -} (Z = 8) to Mg^{2 +} (Z = 12).$
$Hydration energy is directly proportional to the charge-to-size ratio, which increases as the ionic radius decreases and the ionic charge increases.$
$Hence the answer is (B).$

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