Why do halogen reactivities decrease down the group?

Halogen reactivities decrease down the group due to an increase in atomic size and a decrease in effective nuclear charge.

In more detail, the reactivity of halogens, which are found in Group 7 of the Periodic Table, decreases as you move down the group from fluorine to iodine. This is primarily due to two key factors: an increase in atomic size and a decrease in effective nuclear charge.

As you move down the group, the number of electron shells in each halogen atom increases. This results in a larger atomic radius, meaning the outermost electrons are further away from the nucleus. The increased distance between the nucleus and the outermost electrons weakens the attractive force between them. This makes it more difficult for the atom to gain an extra electron, which is the main way halogens react.

In addition to this, the effective nuclear charge, which is the net positive charge experienced by an electron in a multi-electron atom, decreases down the group. This is because the inner electron shells shield the outermost electrons from the full positive charge of the nucleus. As a result, the outermost electrons are less strongly attracted to the nucleus, further reducing the atom's ability to attract and gain an extra electron.

These two factors combined explain why halogen reactivity decreases down the group. Fluorine, at the top of the group, is the most reactive halogen because it has the smallest atomic size and the highest effective nuclear charge. Conversely, iodine, at the bottom of the group, is the least reactive halogen due to its larger atomic size and lower effective nuclear charge.