The reactivity of alkali metals increases as we move down the group. As the atomic number increases, the atomic racfcus also increases and the single electron in the valence shell is located further away from the nucleus. Hence, Li is least reactive and Francium is the most reactive. In case of halogens, the reactivity gradually decreases as we move down the group. All of them require one electron to complet, the outermost shell and the nuclear attraction is greater on the nearest shell (i.e., Ilnd shell in F) than in the fathest shell (i.e., vith shell in iodine). Hence, F is most reactive and I is least reactive.
