Question

The covalent alkaline earth metal halide $(X=Cl,Br,I)$ is

• A. $Sr{X}_2$
• B. $Ca{X}_2$
• C. $Mg{X}_2$
• D. $Be{X}_2$

Answer 1

The correct option is D $Be{X}_2$

According to Fajan's rule, degree of covalency (ionic potential), $\varphi \propto -$ polarisation power of the cation $\propto$ charge on the cation.
$\propto \frac{1}{\text{size of the cation}}$
Alkaline earth metals contains bipostive $\left(H^{2+}\right)$ ions in their compounds.
So, here
(i) Charge on cation, i.e. + 2 is constant.
(ii) Halide present $\left(X^{-}\right)$ is also constant.
So, the covalent character depends on the size of alkaline earth metal. As we move down the group, size of metal ion increases.
$B{e}^{2+}<M{g}^{2+}<C{a}^{2+}<S{r}^{2+}<B{a}^{2+}$
So, $B{e}^{2+}$ readily forms covalent compounds like $Be{X}_2$, because of very high positive charge density over its small size, so that it readily polarises anionic spherical electron cloud.