The increasing order of reactivity among group elements is Li < Na < Rb < Cs whereas that among group 17 elements is F > Cl > Br > I. Explain

As ionisation enthalpy decreases from top to bot-tom in group , therefore, the reactivity increases down the group from Li to Cs.
The group 17 elements are halogens with electronic configuration of n${{s}^{2}}$n${{p}^{5}}$ in their valence shells, they try to become n${{s}^{2}}$n${{p}^{6}}$ by acquiring one electron.
All the halogens are highly reactive elements due to the following reasons :

(i) Low bond dissociation energy :
Halogens have low bond dissociation energy. ${{F}{2}}$ has lowest bond dissociation energy which makes it the most reactive.
(ii) High electron enthalpy : Due to high electron enthalpy these elements have a strong tendency to gain an electron and thus are highly reactive.
(iii) High reduction potentials : ${{F}
{2}}$ has maximum
and ${{I}_{2}}$ minimum reduction potentials. Hence order of reactivity is F > Cl > Br > I.