Q. Consider the following abbreviations for hydrated alkali ions. $X = \left\lbrack Li\left( H_{2}O \right)_{n} \right\rbrack^{+}$ $Y = \left\lbrack K\left( H_{2}O \right)_{n} \right\rbrack^{+}$ $Z = \left\lbrack Cs\left( H_{2}O \right)_{n} \right\rbrack^{+}$ What is the correct order of size of these hydrated alkali ions?
A) $X > Y > Z$
B) $Z > Y > X$
C) $X = Y = Z$
D) $Z > X > Y$
Explanation
(a)
All alkali metal salts are soluble in water. The degree of hydration depends upon the size of the cation. Smaller the size of cation, greater is its charge density and hence, greater is its tendency to withdraw electrons from molecules which are thus polarised. $Li^{+}$ ion being smallest in size among alkali metal ions is the most extensively hydrated while $Cs^{+}$ ion the largest alkali metal ion is the least hydrated. The size of hydrated alkali ions is as
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ Li^{+} > Na^{+} > K^{+} > Rb^{+} > Cs^{+}$
(Relative ionic radii in water)
(Relative degree of hydration)
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.