JEE Main · 2006 · 30 Apr 2006 · Offline · s-Block Elements

The ionic mobility of alkali metal ions in aqueous solution is maximum for

(A) $K^{+}$
(B) $Rb^{+}$ ✓ Correct
(C) $Li^{+}$
(D) $Na^{+}$

Answer: (B)

Solution

Ionic mobility in water depends not on the bare ion but on its hydrated size, since a smaller, more highly charged ion drags a thicker shell of water with it. Lithium, though smallest as a bare ion, has the greatest charge density and so the largest hydration sphere and the lowest mobility; moving down to rubidium the bare size grows but the hydration shell thins out steadily. Rubidium therefore ends up with the smallest hydrated radius of the series and the maximum ionic mobility, so

R b^{+}

moves fastest.

JEE Main · 2006 · 30 Apr 2006 · Offline · Single correct option

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