Here's why:

* Halogens are in Group 17 (VIIA) of the periodic table. This means they have 7 electrons in their outermost energy level.

* Valence electrons are the electrons in the outermost energy level of an atom. They are the ones involved in chemical bonding.

Therefore, all halogens (like fluorine, chlorine, bromine, iodine, and astatine) have 7 valence electrons.