Question

Consider the following abbreviations for hydrated alkali ions $X={\left[Li{\left(H_{2}O\right)}_n\right]}^{+}$ $Y={\left[K{\left(H_{2}O\right)}_n\right]}^{+}Z={\left[Cs{\left(H_{2}O\right)}_n\right]}^{+}$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. $Z>X>Y$

Answer 1

Answer: (A)

All alkali metal salts are soluble in water. The degree of hydration depends upon the size of the cation. Smaller the size of a cation, greaten is its charge density and hence, greater is its tendency to withdraw electrons molecules which are thus polarised. $L{i}^{+}$ ion being smallest in size among alkali metal ions is the most extensively hydrated while $C{s}^{+}$ ion the largest alkali metal ion is the least hydrated. The size of hydrated alkali ions is as
$L{i}^{+}>N{a}^{+}>K^{+}>R{b}^{+}>C{s}^{+}$
(Relative ionic radii in water) (Relative degree of hydration)