The sBlock Elements - Result Question 7
####7. The sequence of ionic mobility in aqueous solution is :
[2008]
(a) $K^{+}>Na^{+}>Rb^{+}>Cs^{+}$
(b) $Cs^{+}>Rb^{+}>K^{+}>Na^{+}$
(c) $Rb^{+}>K^{+}>Cs^{+}>Na^{+}$
(d) $Na^{+}>K^{+}>Rb^{+}>Cs^{+}$
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Solution:
- (b) Smaller the ion more is its ionic mobility in aqueous solution. Ionic radii of the given alkali metals is in the order $Na^{+}<K^{+}<Rb^{+}<Cs^{+}$and thus expected ionic mobility will be in the same order $Cs^{+}<Rb^{+}<K^{+}<Na^{+}$. However due to high degree of solvation (or hydration) and because of lower size or high charge density, the hydrated ion size follows the same order $Cs^{+}<Rb^{+}<K^{+}<Na^{+}$and thus conductivity order is $Cs^{+}>Rb^{+}>K^{+}>Na^{+}$.
(d) The stability of alkali metal hydrides decreases from $Li$ to Cs. It is due to the fact that $M-H$ bonds becomes weaker with increase in size of alkali metals as we move down the group from $Li$ to Cs. Thus the order of stability of hydrides is
$LiH>NaH>KH>RbH>CsH$
