The elements present in group 1 have only 1 valence electron, which they tend to lose.

Group 17 elements, on the other hand, need only one electron to attain the noble gas

configuration. On moving down group 1, the ionization enthalpies decrease. This means

that the energy required to lose the valence electron decreases. Thus, reactivity increases

on moving down a group. Thus, the increasing order of reactivity among group 1 elements

is as follows: Li < Na < K < Rb < Cs

In group 17, as we move down the group from Cl to I, the electron gain enthalpy becomes

less negative i.e., its tendency to gain electrons decreases down group 17. Thus, reactivity

decreases down a group. The electron gain enthalpy of F is less negative than Cl. Still, it

is the most reactive halogen. This is because of its low bond dissociation energy. Thus, the

decreasing order of reactivity among group 17 elements is as follows: F > Cl > Br > I