Question

Which is polymeric hydride?
A. $Ca{H_2}$
B. $Mg{H_2}$
C.$\;Ba{H_2}$
D. $\;Sr{H_2}$

Answer 1

Hydrides are a compound where hydrogen is the negative part of the molecule. I.e. Hydrogen is as ${H^ - }$. Hydrides can be ionic or covalent. In ionic hydride, bond formation takes place by electron donation and acceptance. But in covalent compound bond formation takes place by sharing of electrons.

Complete step by step answer:
When the hydride molecules get joined and form a polymeric structure then they are known as a polymeric hydride.
In the case of $Mg{H_2}$ magnesium forms a normal bond with hydrogen to form hydride. But in this molecule, the total valence electron is only four. Therefore, the octet is not fulfilled yet, that is magnesium is electron deficient in $Mg{H_2}$ . Due to this reason, to gain octet fulfilled stability $Mg{H_2}$ undergoes polymerization by forming a hydrogen bridge bond between two magnesium atoms.
The structure of $Mg{H_2}$ is shown below,
This kind of behavior is also shown in $\;Be{H_2}$ .

So, the correct option is B.

Note:
Electron deficient means, in the molecule there is a lack of electrons. Ionic halide is called saline. If the octet is fulfilled then it is called electron precise. If there is a lone pair on the molecule then it is called electron-rich. And any hydride of transition metal is an interstitial halide.