Among the alkali metal ions in aqueous solution, which has the largest hydrated radius?

A $Na X^{+}$
B $Li X^{+}$
C $Cs X^{+}$
D $K X^{+}$

Answer: B

The smallest bare ion,

Li X^{+}

, has the highest charge density and binds the largest number of water molecules. Its hydrated radius is therefore the largest, and hydrated radii fall in the order

Li X^{+} > Na X^{+} > K X^{+} > Cs X^{+}

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