The elements of group 1 have only one electron in their respective valence shells and thus have a strong tendency to lose this electron. The tendency to lose electrons, in turn, depends upon the ionization enthalpy. Since the ionization enthalpy decreases down the group, therefore, the reactivity of group 1 elements increases in the same order: \(\mathrm{Li} < \mathrm{Na} < \mathrm{K} < \mathrm{Rb} < \mathrm{Cs}\). In contrast, the elements of group 17, have seven electrons in their respective valence shells and thus have a strong tendency to accept one more electron. The tendency to accept electrons, in turn, depends upon their electrode potentials. Since the electrode potentials of group 17 elements decrease in the order: \(\mathrm{F}(+2.87 \mathrm{~V})>\mathrm{Cl}(1.36 \mathrm{~V}), \mathrm{Br}(1.08 \mathrm{~V})\) and \(\mathrm{I}\) \((+0.53 \mathrm{~V})\), therefore, their reactivity also decrease in the same order : \(\mathrm{F}>\mathrm{Cl}>\mathrm{Br}>\mathrm{I}\).