Why Halogens are not identical?

Answer 1

The atomic weights are different. This means more protons and neutrons in the nucleus and thus more total electrons. This in turn means the outer electrons are further away from the nucleus and not as tightly bound which cahnges the reactivity and electronegativity.

Answer 2

Halogens such as fluorine, chlorine, bromine, iodine, and astatine are part of the same group in the Periodic Table but have different atomic numbers and thus different numbers of protons, neutrons, and electrons. This leads to differences in their physical and chemical properties, despite sharing similar characteristics such as being highly reactive nonmetals.