How does the reactivity change for halogens as you go down Group 7?

As you move down Group 7 of the periodic table, which consists of the halogens (fluorine, chlorine, bromine, iodine, and astatine), the reactivity of these elements decreases. This trend can be explained by the atomic structure of the halogens.

The reactivity of halogens is primarily related to their ability to gain an electron to achieve a stable noble gas configuration. As you proceed down the group, the atomic size increases due to the addition of electron shells. This increase in distance between the nucleus and the outermost electrons reduces the nucleus's effective attraction for incoming electrons. Consequently, larger halogen atoms are less effective at attracting and gaining an additional electron compared to smaller halogens.

In summary, the decrease in reactivity down Group 7 can be attributed to the larger atomic size and the resulting diminished nuclear attraction for additional electrons, making it harder for heavier halogens to react and form stable compounds.