Question

Which of the following ions has maximum mobility in aqueous solutions?

• A. ${\text{Rb}}^{\text{+}}$
• B. ${\text{Cs}}^{\text{+}}$
• C. $L{i}^{\text{+}}$
• D. $N{a}^{+}$

Answer 1

The correct option is C $L{i}^{\text{+}}$

The alkali metal ion exists as hydrated ions $M^{+}(H_{2}O{)}_n$ in the aqueous solution. The degree of hydration decreases with increasing ionic size as we go down the group. Hence, Li+ ion is mostly hydrated e.g. $\left[Li\right(H_{2}O{)}_6{]}^{+}$. Since the mobility of ions is inversely proportional to the size of their hydrated ions, hence the increasing order of ionic mobility is

$L{i}^{+}<N{a}^{+}<K^{+}<R{b}^{+}<C{s}^{+}$

The correct option is $A$.