Arrange Na+,Mg2+ and Al3+ in increasing order of energy of hydration:
A
Na+<Mg2+<Al3+
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B
Al3+<Mg2+<Na+
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C
Al3+<Na+<Mg2+
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D
Mg2+<Al3+<Na+
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Solution
The correct option is DNa+<Mg2+<Al3+ The hydration enthalpies (ie, energies of hydration) of metal ions decreases with increase in ionic radii. Because of the smaller size of Al3+ than Mg2+ and Na+, its energy of hydration larger than both these ions. The order of heat of hydration is Al3+>Mg2+>Na+