The most active element in group 17 (the Halogens) is fluorine. This is due to the fact that fluorine has the smallest atomic radius of the halogens, which gives it the lowest electron affinity. Fluorine's electrons are less attracted to its nucleus, enabling it to be more active in forming bonds with other elements.
Group 17 comprises of five elements: fluorine, chlorine, bromine, iodine and astatine. All five elements are toxic nonmetals with seven valence electrons. The halogens exist as diatomic molecules in the form of X2. Unlike the other halogens, astatine is radioactive. It is included in this group because it shares other properties with the halogens.