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 outermost shells and therefore, have strong tendency to lose this electron. The tendency of these elements to lose the valence electron depends upon the ionization enthalpy. Since ionisation enthalpy decreases down the group, therefore, the reactivity of group 1 elements increase in the same order `Li lt Na lt K lt Rb lt Cs.`
On the other hand, the elements of group 17 have seven electrons in their respective valence shells and therefore, they have strong tendency to acept 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. 86V, Cl = + 1. 36 V, Br =- 1. 08 V and I = + 0.53V) and ` therefore, their reactivities also decrease in the same order as `F gt Cl gt Br gt I.`