Which of the alkaline earth metals cation has least hydration enthalpy ?

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Mg²⁺

Be²⁺

Ba²⁺

Ca²⁺

answer is D.

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Detailed Solution

Be²⁺ > Mg²⁺ > Ca²⁺ > Sr²⁺ > Ba²⁺: Size of isolated M⁺²ions
Be²⁺ > Mg²⁺ > Ca²⁺ > Sr²⁺ > Ba²⁺: Degree of hydration
Be²⁺ > Mg²⁺ > Ca²⁺ > Sr²⁺ > Ba²⁺: Hydration Enthalpies
$\large \xrightarrow{{\;\;Be_{(aq)}^{2 + } > {\text{ }}Mg_{(aq)}^{2 + } > {\text{ }}Ca_{(aq)}^{2 + } > {\text{ }}Sr_{(aq)}^{2 + } > {\text{ }}Ba_{(aq)}^{2 + }\;\;}}\;:\;Size\;of\;hydrated\;ions$
As the size of M⁺² ions [isolated] increases effective nuclear charge on the ion decreases nuclear attractive forces on the water molecules decreases, number of water molecules surrounded around the M⁺² ion decreases degree of hydration decreases hydration enthalpy decreases
$\large Degree\;of\;hydration\; \propto \;hydration\;energy\; \propto \;\frac{1}{{Size\;of\;ion\;\left[ {isolated} \right]}}$