Question

Which of the following hydrides are ionic?
A.$Ca{H}_{2}$
B.$Ba{H}_{2}$
C.$Sr{H}_{2}$
D.$Be{H}_{2}$

Answer 1

Hint: Ionic hydrides are the compounds that are formed between hydrogen and most active metals in the periodic series. These are defined by the presence of a hydride which is a negatively charged hydrogen ion $({H}^{-})$.

Complete step by step answer:
Ionic hydrides are also known as saline hydrides or as pseudohalides. These are stoichiometric compounds of hydrogen. They are formed when the hydrogen molecule, the hydride ion, reacts with highly electropositive s-block elements i.e., alkali metals and alkaline earth metals. Mainly group 1 and group 2 elements form ionic hydrides.

Now, among the given compounds, $Ca{H}_{2}$, $Ba{H}_{2}$, $Sr{H}_{2}$, and $Be{H}_{2}$, the central atoms are Ca, Ba, Sr and Be. Ca, Ba, and Sr form ionic hydrides as they are highly electropositive elements. Thus, options (a), (b), and (c) are correct options.

But, despite being an electropositive element beryllium forms and covalent bond with hydrogen rather than forming an ionic hydride. The hydride is not ionic because of the lower electronegativity difference in comparison with the other s-block elements. Also, Be is small with relatively high charge and the ${H}^{-}$ ion is a large anion, due to these differences in the sizes, Be ends up having a covalent bond with hydrogen.

Note: It is not just Be that does not form an ionic hydride, Mg also does not form an ionic hydride. It is also bonded with hydrogen through a covalent bond. The reasons why this happens are the same as that of beryllium.