Question

The metal which has the greatest tendency for the reaction
M in grams + M in aqueous+ electron
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Answer 1

Among alkali metals, Rb has the lowest ionization energy. Hence it has maximum tendency to lose an electron and form a cation. 

Hence Rb has greatest tendency for the reaction M(g)Maq++e−.

Note; In the group of alkali metals, the ionization energy decreases on moving from top to bottom.