sblock Elements 2 Question 7
7. The covalent alkaline earth metal halide $(X=\mathrm{Cl}, \mathrm{Br}, \mathrm{I})$ is
(2019 Main, 8 April II)
(a) $\operatorname{Sr} X_{2}$
(b) $\mathrm{Ca} X_{2}$
(c) $\operatorname{Mg} X_{2}$
(d) $\mathrm{Be} X_{2}$
Show Answer
Answer:
Correct Answer: 7. (a)
Solution:
- Key Idea According to Fajan’s rule, degree of covalency (ionic potential), $\phi \propto$-polarisation power of the cation $\propto$ charge on the cation
$$ \propto \frac{1}{\text { size of the cation }} \text {. } $$
Alkaline earth metals contains bipositive $\left(\mathrm{H}^{2+}\right)$ 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.
$$ \mathrm{Be}^{2+}<\mathrm{Mg}^{2+}<\mathrm{Ca}^{2+}<\mathrm{Sr}^{2+}<\mathrm{Ba}^{2+} $$
$\mathrm{So}, \mathrm{Be}^{2+}$ readily forms covalent compounds like $\mathrm{Be} X_{2}$, because of very high positive charge density over its small size, so that it readily polarises anionic spherical electron cloud.
