# The ease of adsorption of the hydrated alkali metal ions on an ion- exchange resins follow the order:A.${K^ + } < N{a^ + } < R{b^ + } < L{i^ + }$B.$N{a^ + } < L{i^ + } < {K^ + } < R{b^ + }$C.$L{i^ + } < {K^ + } < N{a^ + } < R{b^ + }$D.$R{b^ + } < {K^ + } < N{a^ + } < L{i^ + }$

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Hint:The ease of adsorption of hydrated metal ions depends on the size of the hydrated ion. As the size increases, the ease of adsorption also increases. To answer this question, you must recall the trend of change in the atomic radius of an atom in a periodic table. As we move from left to right in a period, the atomic size decreases and when we move down the group, an increase in the size is observed.

As we move down in the group 1, the atomic size increases with the increase in the number of shells in the atoms. Similarly the ionic size also shows an increase as we move down the group. So, the order of size of the ions can be given as, $L{i^ + } < N{a^ + } < {K^ + } < R{b^ + }$
Smaller is the size of a cation, higher is the charge density on it and thus, higher is the number of water molecules that attach to it in an aqueous solution. As a result, the sizes of the hydrated ions follow the order, $R{b^ + } < {K^ + } < N{a^ + } < L{i^ + }$.