Give reason for the decreasing order of the conductivity of the following.
`Cs_((aq))^(o+)gtRb_((aq))^(o+)gtK_((aq))^(o+)gtNa_((aq))^(o+)gtLi_((aq))^(o+)`

Explanation

In aqueous medium. Ions are hydrated. `Li^(o+)` ion, because of its maximum charge to radius ratio, has maximum positive charge density and thus is heavily hydrated. On moving form `Li^(o+)` ion to `Cs^(o+)` ion, the degree of hydration decreases. As a result, the `Li^(o+)` ion is maximum hydrated and the the `Cs^(o+)` ion is least hydrated. Consequently, the size of the hydrated `Li^(o+)` ion is maximum and it has the least mobility while the size of the `Cs^(o+)` ion is minimum and it has the maximum mobility.
`Cs_((aq))^(o+)gtRb_((aq))^(o+)gtK_((aq))^(o+)gtNa_((aq))^(o+)gtLi_((aq))^(o+)`