Question

Which of the following hydrides is the most stable?
A.$LiH$
B.$KH$
C.$NaH$
D.$CsH$

Answer 1

Hydrides are chemical compounds which are anion of hydrogen. They have less electronegative elements. Thus, when a hydrogen atom reacts with any compound then the product is said to be a hydride. Hydrides are divided into various categories on the basis of chemical bonding. We can approach this question using the information related to the size of metal atoms available.

Complete step by step solution: We know that when different compounds react with hydrogen then the product obtained is known as a hydride. Hydrogen reacts with many elements except noble gases that do not form hydrides. The properties vary depending on the type of intermolecular force that exists between elements, molecular mass, temperature and various other factors.
There are three types of hydrides depending on the chemical bonding- ionic, covalent and metallic. Ionic hydrides are formed when hydrogen reacts with highly electropositive $s -$ block elements.
$2{X_{\left( s \right)}} + {H_2} \to 2X{H_{\left( s \right)}}$
${X_{\left( s \right)}} + {H_2}_{\left( g \right)} \to X{H_2}_{\left( s \right)}$
Covalent hydrides are formed when hydrogen reacts with similar electronegative elements like$Si$.
${H_{2\left( s \right)}} + C{l_{2\left( g \right)}} \to 2HC{l_{\left( g \right)}}$
Metallic hydrides are hydrogen compounds forming bonds with other metal elements.
The reactivity order of the given elements-$Li,Na,K$ and $Cs$is:
$Li < Na < K < Cs$
So, Li is the most stable element as it is the least reactive element. Also as we go down the group, the size of the metal increases and the $X - H$ bond becomes weaker. Thus, $LiH$is the most stable hydride among$LiH$,$NaH$,$CsH$and$KH$.

Thus the correct answer to the given question is option (1).

Note: On the basis of size, $LiH$ is the most stable hydride. Hydrides are used as reducing agents in many chemical industries and are also used as drying agents. They are used as strong bases in organic synthesis. Hydrides further react with water to form hydrogen and hydroxide salt. In hydrides, hydrogen has an oxidation number equal to -1.