Question #085e9

1 Answer
Ernest Z.
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:

#"2M" + "H"_2 →"2M"^"+" "H"^"-"#

Atomic size

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.

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