Question

Nuclear attraction is often the deciding control factor for the association of natural molecules to
a given metal ion. Which one of the following represents the correct order of stability of the ions?
$\left[Be\right(H_{2}O{)}_4{]}^{2+},\left[Mg\right(H_{2}O{)}_4{]}^{2+},Ca\left(H_{2}O{)}_4{]}^{2+}andSr\right(H_{2}O{)}^4{]}^{2+}$

• A. $\left[Be\right(H_{2}O{)}_4{]}^{2+}>Sr\left(H_{2}O{)}_4{]}^{2+}\right]>\left[Mg\right(H_{2}O{)}_4{]}^{2+}>\left[Ca\right(H_{2}O{)}^4{]}^{2+}$
• B. $\left[Ca\right(H_{2}O{)}_4{]}^{2+}>\left[Mg\right(H_{2}O{)}_4{]}^{2+}>\left[Be\right(H_{2}O{)}_4{]}^{2+}>Sr(H_{2}O{)}_4{]}^{2+}$
• C. $\left[Sr\right(H_{2}O{)}_4{]}^{2+}>\left[Ca\right(H_{2}O{)}_4{]}^{2+}>\left[Mg\right(H_{2}O{)}_4{]}^{2+}>\left[Be\right(H_{2}O{)}_4{]}^{2+}$
• D. $\left[Be\right(H_{2}O{)}_4{]}^{2+}>\left[Mg\right(H_{2}O{)}_4{]}^2>\left[Ca\right(H_{2}O{)}_4{]}^{2+}>Sr(H_{2}O{)}_4{]}^{2+}$

Answer 1

Answer: (D)

$\left[Be\right(H_{2}O{)}_4{]}^{2+}>\left[Mg\right(H_{2}O{)}_4{]}^2>\left[Ca\right(H_{2}O{)}_4{]}^{2+}>Sr(H_{2}O{)}_4{]}^{2+}$

The degree of hydration and the amount of hydration energy decreases as the size of the ion increases from Be^{2+} to Sr^{2+}.

$B{e}^{2+}<M{g}^{2+}<C{a}^{2+}<S{r}^{2+}$

Hydration -2494 - 1921 -1577 -1443

Ergy (kJ moI${}^{-1})$

Thus, stability of hydrated ion is

$\left[Be\right(H_{2}O{)}_4{]}^{2+}>\left[Mg\right(H_2{O}_4{]}^{2+}>\left[Ca\right(H_{2}O{)}_4{]}^{2+}>\left[Sr\right(H_{2}O{)}_4{]}^{2+}$