Question

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.

Answer 1

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:
$F > Cl > Br > I$