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The sequence of ionic mobility in aqueous solutions is -
A
$Rb^+ \gt K^+ \gt Cs^+ \gt Na^+$
B
$Na^+ \gt K^+ \gt Rb^+ \gt Cs^+$
C
$K^+ \gt Na^+ \gt Rb^+ \gt Cs^+$
D
$Cs^+ \gt Rb^+ \gt K^+ \gt Na^+$
Detailed Solution
Smaller the size of the cation, higher is its charge density and higher will be its hydration.
Extent of hydration: $Na^+ \gt K^+ \gt Rb^+ \gt Cs^+$
A more heavily hydrated ion carries a larger shell of water molecules, so its effective (hydrated) size increases.
Hydrated radius: $Na^+(aq) \gt K^+(aq) \gt Rb^+(aq) \gt Cs^+(aq)$
A bigger hydrated ion moves more slowly, so its mobility in aqueous solution decreases.
Hence the ionic mobility follows the sequence: $Cs^+ \gt Rb^+ \gt K^+ \gt Na^+$
Extent of hydration: $Na^+ \gt K^+ \gt Rb^+ \gt Cs^+$
A more heavily hydrated ion carries a larger shell of water molecules, so its effective (hydrated) size increases.
Hydrated radius: $Na^+(aq) \gt K^+(aq) \gt Rb^+(aq) \gt Cs^+(aq)$
A bigger hydrated ion moves more slowly, so its mobility in aqueous solution decreases.
Hence the ionic mobility follows the sequence: $Cs^+ \gt Rb^+ \gt K^+ \gt Na^+$