Which element in group 7 is the least reactive?

Iodine is considered the least reactive element in group 7, also known as the halogens. Reactivity in the halogen group decreases as you move down the group from fluorine to iodine. This trend is due to the increasing atomic size, which results in a greater distance between the valence electrons and the nucleus. The larger atomic radius in iodine means that its outer electrons are held less tightly compared to those of the smaller halogens like fluorine, chlorine, and bromine.

This makes iodine less likely to gain an electron to form a negative ion, which is the characteristic behavior of reactive halogens. In practical terms, iodine's reactivity is significantly lower, which is observed in its chemical behavior and reaction with other elements. For example, while fluorine and chlorine readily react with many substances, iodine tends to react more sluggishly, only forming compounds under specific conditions. This lack of reactivity is a defining characteristic of iodine compared to its counterparts in group 7.