The increasing order of reactivity among group `1` elements is `LiltNaltKltRbltCs` whereas that among group 17 elements is `FgtClgtBrgtI`. Explain.

Explanation

The elements of group 1 have only one electron in their respective valence shells and thus 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 : `Li lt Na lt K lt Rb lt 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 decreases in the order : `F (+ 2.87 V ) gt C1 ( + 1.36 V ) , Br ( 1.08 V )` and `I (+ 0.53 V)` therefore their reactivities also decreases in the same order `: F gt C1 gt Br gt I`
Alternatively tendency to accept electrons can be linked to electron gain enthalpy . since electron gain enthalpy becomes less and less negative as we move from C1 to I therefore reactivity increases from to I, F , however is the most reactive due to its low bond dissociation energy .