Which alkali metal is the softest?

1. Softness for alkali metals is attributed to large size atoms and weak intermolecular forces of attraction.
2. The size increases as we move down from top to bottom due to the weak forces of attraction.
3. This means that the reactivity increases as electrons can be easily removed from the valence shell.
4. Therefore, softness increases down the group and follows the order : ${\mathrm{Na}}^{+}>{\mathrm{K}}^{+}>{\mathrm{Rb}}^{+}>{\mathrm{Cs}}^{+}$.
5. Lithium shows anomalous behaviour and is much harder.
6. It is due to its exceptionally small size and high polarising power.