TOP to bottom because of an increase in the atomic size. Alkali metals have one electron in their respective valence shells.They have a STRONG tendency to lose this electron and acquire the stable configuration of the nearest noble gas.Thus, the reactivity of alkali metals depends upon their ability to lose ELECTRONS.Since their tendency to lose electrons increases down the group hence their reactivity increases down the group. In contrast, HALOGENS have seven electrons in their respective valence shells and thus have a strong tendency to acquire or gain one electron to achieve the stable electronic configuration of the nearest noble gas.Thus, the reactivity of halogens depends upon their ability to attract electrons.Since this tendency to gain electrons decreases down the group due to an increase in size hence reactivity decreases down the group.Moving down a group in the periodic table, the number of electrons and filled electron shells increases, but the number of valence electrons remains the same.The outermost electrons in a group are exposed to the same effective nuclear charge, but electrons are FOUND farther from the nucleus as the number of filled energy shells increases. Therefore, the atomic radii increase.