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

Options

A. `NaHgtLiHgtKHgtRbHgtCsH`
B. `LiHgtNaHgtKHgtRbHgtCsH`
C. `CsHgtRbHgtKHgtNaHgtLiH`
D. `KHgtNaHgtLiHgtCsHgtRbH`

Answer

B

Explanation

Down the group, atoms of alkali metals increases in ionic radii. This results in the charge density of their corresponding cations decreasing down the group. Thus, as we go down the group, the cations become ..softer...
The hydride ion is ..hard.., having high negative charge density. Applying the ..hard-soft acid-base.. (HSAB) principle, we would expect the ..hard.. ions to form more stable compounds with each other.
Hence the stability of hydrides decreases down the group.