Comment of the following observations : The mobilities of the alkali metal ions in aqueous solution are Li^(+) lt Na^(+) lt K^(+) lt Rb^(+) lt Cs^(+)

Comment of the following observations : The mobilities of the alkali

1.	Comment of the following observations : The mobilities of the alkali metal ions in aqueous solution are Li^(+) lt Na^(+) lt K^(+) lt Rb^(+) lt Cs^(+)
Answer» Solution : On moving down the alkali group, the ionic and atomic SIZES of the metals increase. The given alkali metal ions can be arranged in the INCREASING order of their ionic sizes as: `Li^(+) lt Na^(+) lt K^(+) lt Rb^(+) lt Cs^(+)` Smaller the size of an ion, the more highly is it HYDRATED. Since `Li^(+)`is the smallest, it gets heavily hydrated in an aqueous solution. On the other hand, `Cs^(+)`is the LARGEST and so it is the least hydrated. The given alkali metal ions can be arranged in the DECREASING order of their hydrations as : `Li^(+) gt Na^(+) gt K^(+) gt Rb^(+) gt Cs^(+)` Greater the mass of a hydrated ion, the lower is its ionic mobility. Therefore, hydrated `Li^(+)`ion is the least mobile and hydrated `Cs^(+)`ion is the most mobile. Thus, the given alkali metal ions can be arranged in the increasing order of their mobilities as : `Li^(+) lt Na^(+) lt K^(+) lt Rb^(+) lt Cs^(+)`