International General Certificate of Secondary Education (IGCSE) Chemistry Practice Exam

Fluorine is the most reactive element in Group 7, which consists of the halogens. This reactivity is primarily due to its atomic structure. Fluorine has seven electrons in its outer shell and requires only one additional electron to achieve a stable octet configuration. As the most electronegative element, fluorine has a strong tendency to attract electrons from other atoms, enabling it to form compounds readily and complete its outer shell.

The reactivity of halogens decreases down the group. Chlorine, while also reactive, is not as strong an electron attractor as fluorine. Bromine is even less reactive due to its larger atomic size and the increased distance of the outer electrons from the nucleus, leading to reduced attraction for incoming electrons. Iodine, positioned below bromine in the group, exhibits even lower reactivity for similar reasons, as it has even more electron shielding and greater atomic size, resulting in a weaker ability to attract electrons.

Thus, fluorine's high electronegativity and smaller atomic radius make it the most reactive halogen in Group 7.