Question

Out of LiH, $Mg{H_2}$and CuH.
(A)- All are ionic hydrates.
(B)- LiH, $Mg{H_2}$ are ionic and CuH is covalent hydride.
(C)- All are covalent hydrides.
(D)- LiH and $Mg{H_2}$are ionic hydride and CuH is intermediate hydride.

Answer 1

The above question is based on the formation of hydrides of different elements. Dihydrogen can react to all the elements under certain conditions i.e. at a specific temperature and pressure, except noble gases and it forms binary compounds known as hydrides.

Complete step by step answer:
Since hydrogen is an exception among all the elements present in the periodic table, it’s properties also vary when it gets reacted with different elements. Instead of being placed on the top of the periodic table, its placement is still a subject of discussion. It can show both positive and negative oxidation states depending on the electronegativity of the element it is going to react. Therefore, it can act as both oxidizing and reducing agent.
The hydrides are classified into three categories:
- Ionic or saline or salt like hydrides: These are compounds of hydrogen formed with most of the s-block elements which are highly electropositive in nature. The ionic hydrides are crystalline, nonconducting and non-volatile in solid state. However, their melts can conduct electricity and on electrolysis liberate hydrogen gas at anode, which confirms the existence of H- ion.
$2H{\text{ }}\left( {melt} \right){\text{ }} - - - > {H_2}{\text{ }}\left( {gas} \right){\text{ }} + {\text{ }}2{e^ - }$
- Covalent or molecular hydrides: Dihydrogen forms molecular compounds with almost every p-block element. They are volatile in nature as the bonds formed are covalent bonds.
- Metallic or non-stoichiometric hydrides: These are formed by elements of d and f block. These hydrides conduct heat and electricity (not as efficiently as their parent metals do). Unlike saline hydrides, they are almost nonstoichiometric i.e. they are deficient in hydrogen.
From the above discussion, it is clear that LiH and $Mg{H_2}$are ionic in nature since they are s-block elements and CuH is an intermediate hydride since Cu is a d-block element.

Therefore, the correct answer is (D)- LiH and $Mg{H_2}$are ionic hydride and CuH is intermediate hydride.

Note: There are many exceptions and special conditions in this trend. For example, Diborane (${B_2}{H_6}$) is electron deficient in nature, Elements of group 15-17 form compounds which are electron rich in nature like $N{H_3}$which has 1- lone pair, ${H_2}O$has 2 lone pair. In fact, $Be{H_2}$and $Mg{H_2}$instead of being s-block hydrides are polymeric in structure.