The increasing order of reactivity among group-$1$ elements is $Li < Na < K < Rb < Cs$ whereas that among group-$17$ elements is $F > Cl > Br > I$. Explain.

(A) The elements of group-$1$ have only one electron in their outermost shells and therefore have a strong tendency to lose this electron. The tendency of these elements to lose the valence electron depends upon the ionization enthalpy. Since ionization enthalpy decreases down the group,the reactivity of group-$1$ elements increases in the order $Li < Na < K < Rb < Cs$.
On the other hand,the elements of group-$17$ have seven electrons in their respective valence shells and therefore have a strong tendency to accept one more electron. The tendency to accept additional electrons depends upon the electrode potentials of group-$17$ elements. The electrode potential of group-$17$ elements decreases from $F$ to $I$ $(F = +2.86 \ V, Cl = +1.36 \ V, Br = +1.08 \ V, I = +0.53 \ V)$ and therefore,their reactivities also decrease in the order $F > Cl > Br > I$.