The second ionisation enthalpy of calcium is more that of first and yet calcium forms $\mathrm{MgCl}_2$ not MgCl . Why?
Answer
This is because, after removing two electrons from Mg , it acquire stable noble gas configuration or in other words, the higher enthalpy of lattice formation of $\mathrm{Mg}^{2+}$ is more than compensates the second ionisation enthalpy requires for the formation of divalent $\mathrm{Mg}^{2+}$ ions.
i. Large cation $\left(\mathrm{K}_{+}\right)$can stabilise large anion $\left(\mathrm{I}^{-}\right)$.
ii. This is because the larger cation $\left(\mathrm{K}^{+}\right)$can stabilize larger anion $\left(\mathrm{Cl}^{-}\right)$.
$\text{BeCl}_2+4\text{H}_2\text{O}\xrightarrow{ \ \ \ \ \ \ }[\text{Be(H}_2\text{O})_4]\text{Cl}_2$
Beryllium chloride being electron deficient in nature get easily hydrolysed.
CaO because others react with $\mathrm{NH}_3 . \mathrm{CaCl}_2$, forms $\mathrm{CaCl}_2 .8 \mathrm{NH} 3$ Conc. $\mathrm{H}_2 \mathrm{SO}_4$ forms $\left(\mathrm{NH}_4\right)_2 \mathrm{SO}_4 . \mathrm{P}_2 \mathrm{O}_5$ forms $\left(\mathrm{NH}_4\right)_3 \mathrm{PO}_4$.
Since, $\mathrm{BeH}_2$ and $\mathrm{BeCl}_2$ have only four electrons in the valence shell, therefore, they are electron deficient molecules. To make up their electron deficiency, each Be atom forms four, three-centre twoelectron bonds or banana bonds. Thus, it is due to electron deficiency that $\mathrm{BeH}_2$ and $\mathrm{BeCl}_2$ have polymeric structures.