The reactivity of halogens decreases as you move down Group 7 of the periodic table. This trend can be attributed to two primary factors: an increase in atomic size and a greater shielding effect.

Halogens include the elements fluorine, chlorine, bromine, iodine, and astatine. As you progress down this group, the reactivity of these elements diminishes. The underlying reasons for this trend are related to changes in atomic structure.

First, let’s consider atomic size. As you descend the group, the atomic size of halogens increases because each successive element has an additional electron shell compared to its predecessor. This increase in size means that the outermost electrons are situated further away from the positively charged nucleus. Consequently, the distance weakens the attractive force between the nucleus and the outermost electrons, making it more difficult for the atom to gain an additional electron to achieve a stable electron configuration. As a result, the reactivity of halogens decreases.

Next, we examine the shielding effect, which significantly influences the reactivity of these elements. The shielding effect occurs when inner shell electrons impede the attractive force of the nucleus on the outer shell electrons. As you move down Group 7, the number of inner electron shells increases, thereby enhancing the shielding effect. This additional shielding further diminishes the attraction between the nucleus and the outermost electrons, reducing the atom’s ability to attract and acquire an extra electron.

In conclusion, the decrease in reactivity of halogens down the group can be attributed to the combined effects of increasing atomic size and the shielding effect. These factors result in a diminished capacity for halogens to gain an extra electron and achieve a stable electron configuration, rendering them less reactive as you move down the group.