The increasing order of reactivity among group `1` elements is `LiltNaltKltRbltCs` whereas that among group 17 elements is `FgtClgtBrgtI`. Explain.

Explanation

The elements of group `1` have only one electron in their respective valence shells thus have a strong tendency to lose this electron. The tendency to lose electrons, in turn, depends upon the `IE`. Since the `IE` decreases down the group, therefore the reactivity of group `1` elements increases in the same order:`LiltNaltKltRbltCs`.
In contrast, the elements of group `17` have seven electrons in their respective valence shells and thus have a strong tendecy to accept one more electron. The tendency to accept electrons, in turn depends upon their electrode potentials. Since the electrode potentials of group `17` elements decrease in the order: `F(+2.87 V)gtCl(+1.36V)`,`Br(1.08 V)` and `I(+0.53 V)`, therefore, their reactivities also decrease in the same order:`FgtClgtBrgtI`.