The increasing order of reactivity among group 1 elements is `Li lt Na lt K lt Rb lt Cs` whereas that among group 17 elements is `F gt CI gt Br gt I`. Explain.

Explanation

The elements of group 1 are electropositive elements and have a tendency to lose their valence electron to form monopositive cations. Their reactivity depends upon the value of ionisation enthalpy. Lower the value of ionisation enthalpy, greater is the reactivity. Since the ionisation enthalpy values decrease on moving down the group, the reactivity of group 1 elements increases on moving down the group, i.e., in the order `Li lt Na lt K lt Rb lt Cs`.
On the other hand, halogens are electronegative elements and have a tendency to gain an electron to form mononegative anions. This tendency is governed by the values of standard electrode potentials. Higher the value of standard electrode potential, greater is the reactivity. Since the values of standard electrode potential decrease on moving down group 17, the reactivity of halogens decreases on moving down the group, i.e., in the order `F gt Cl gt Br gt I`.