Question

Question #085e9

Answer 1

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

Explanation:

All alkali metals react with hydrogen to form metallic hydrides:

`"2M" + "H"_2 →"2M"^"+" "H"^"-"`

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

Thus, a `"Cs"` atom is so big that the `"M"^"+"` ion has little attraction for the `"H"^"-"` ion.

This makes `"Cs"` the least active alkali metal towards hydrogen.