How does the reactivity of halogens change down the group?

The reactivity of halogens decreases down the group from fluorine to astatine.

In the halogen group, which includes fluorine, chlorine, bromine, iodine, and astatine, the reactivity decreases as you move down the group. This is due to the increasing number of electron shells, which results in a greater distance between the nucleus and the outermost electrons.

Fluorine, at the top of the group, is the most reactive halogen. This is because it has the smallest atomic radius, meaning its outermost electrons are closest to the nucleus and therefore strongly attracted to it. This strong attraction allows fluorine to gain an extra electron very easily, making it highly reactive.

As you move down the group, the atomic radius increases due to the addition of electron shells. This means the outermost electrons are further from the nucleus and less strongly attracted to it. For example, chlorine, the next halogen down from fluorine, is less reactive because its outermost electrons are less strongly attracted to the nucleus.

Bromine, iodine, and astatine follow the same pattern, with each being less reactive than the one above it in the group. Astatine, at the bottom of the group, is the least reactive halogen. Its large atomic radius means its outermost electrons are far from the nucleus and weakly attracted to it, making it difficult for astatine to gain an extra electron.

In summary, the reactivity of halogens decreases down the group due to the increasing atomic radius and the corresponding decrease in the attraction between the nucleus and the outermost electrons. This fundamental concept is crucial to understanding the properties and behaviours of the halogens, and indeed, of all elements in the periodic table.