The increasing order of reactivity among group 1 elements is Li < Na < K < Rb is F > CI > Br > I. Explain.

In group 1 elements (alkali metals) the reactivity of the metals is mainly due to the electron releasing tendency of their atoms, which is related to ionisation enthalpy. As we know, the ionisation enthalpy decreases down the group, the reactivity of alkali metals increases.
In group 17 elements (halogens), the reactivity is mainly due to electron gain tendency of the members of the family. Thus reactivity is linked with electronegativity and electron gain enthalpy. As we know, both of these decrease down the group, the reactivity therefore decreases.