What is the order of reactivity of the halogens?

The reactivity of halogens decreases as you move down the group in the Periodic Table. This is because as you move down the group, the outermost shell of the halogen atom gets further from the nucleus and the attraction between the nucleus and the outermost shell decreases. This makes it harder for the atom to attract and gain an electron to achieve a stable electronic configuration.

So, the order of reactivity of halogens (from most reactive to least reactive) is:

1. Fluorine (F)
2. Chlorine (Cl)
3. Bromine (Br)
4. Iodine (I)
5. Astatine (At)

Please note that Astatine is a radioactive element and its chemical properties are not well-studied.