Question #085e9

Sep 10, 2017

It is because ionic size increases from top to bottom in Group 1.

Explanation:

All alkali metals react with hydrogen to form metallic hydrides:

$\text{2M" + "H"_2 →"2M"^"+" "H"^"-}$

The closer you get to the bottom of Group 1, the larger the atom becomes.

Thus, a $\text{Cs}$ atom is so big that the $\text{M"^"+}$ ion has little attraction for the $\text{H"^"-}$ ion.

This makes $\text{Cs}$ the least active alkali metal towards hydrogen.