Why does reactivity decrease when moving down group 7?

Reactivity decreases when moving down group 7, which consists of the halogens, primarily because the outer shell of electrons is further away from the nucleus in the heavier halogens. As you move down the group from fluorine to iodine, the number of electron shells increases. This increase in distance weakens the attraction between the positively charged nucleus and the negatively charged electrons in the outer shell.

The further the outer electrons are from the nucleus, the less influence the nuclear charge has on those electrons. Consequently, the larger halogen atoms are less effective at gaining an additional electron to achieve a stable octet configuration, which is essential for reactivity. In simpler terms, the higher the number of electron shells, the more shielding occurs due to the inner electrons, which reduces the effectiveness of the nucleus in attracting additional electrons.

Other options do not explain the trend in reactivity accurately. The increase in neutron number does not play a direct role in electron shell stability or reactivity. The atomic mass, while it increases down the group, does not directly influence the core behavior of electron gain. Lastly, halogens do not become noble gases, as they are distinctly different groups in the periodic table with their own unique properties. When looking at the behavior of halogens