The alkali metals form salt-like hydrides by the direct synthesis at elevated temperature. The thermal stability of these hydrides decreases in which of the following orders ?

N a H > L i H > K H > R b H > C s S

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L i H > N a H > K H > R b H > C s H

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C s H > R b H > K H > N a H > L i H

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K H > N a H > L i H > C s H > R b H

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Solution

The correct option is A $L i H > N a H > K H > R b H > C s H$
The thermal stability of hydrides of alkali metals decreases down the group as the size of the alkali metals increase, they can be decomposed easily. Smaller is the size of the cation and larger is the size of the anion, stronger is the covalent bond (Fajan's Rule).
So, the correct order of thermal stability of hydrides of alkali metals is $L i H > N a H > K H > R b H > C s H$ .
Hence option B is correct.

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Question 3

Metal hydrides are ionic, covalent or molecular in nature. Among LiH,NaH, KH,RbH,CsH, the correct order of increasing ionic charcter is

(a) LiH > NaH > CsH > KH > RbH

(b) LiH < NaH < KH < RbH < CsH

(d) NaH > CsH > RbH > LiH > KH

L i H, N a H, K H, R b H

C s H

Match the following

A) Thermal stability order of hydrides 1) Na > K >Rb> Li > Cs

B) Ionic nature of hydrides 2) Li > Na > K >Rb> Cs

C) I.P₂ values order 3) CsH>RbH> KH >NaH>LiH

D) Abundance decreasing order 4) LiH>NaH> KH >RbH>CsH

L i H, N a H

C s H

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