The ease of adsorption of the hydrated alkali metal ions on an ion-exchange resin follows the order:
Li+<K+<Na+<Rb+
Rb+<K+<Na+<Li+
K+<Na+<Rb+<Li+
Na+<Li+<K+<Rb+
The larger the sphere of hydration, the easier the adsorption. Hence, the order Rb+<K+<Na+<Li+.
The ionic radii of alkali metal ions in water (hydrated radii) are in the order