Here's why:

* Valence electrons are the electrons in the outermost shell of an atom, and they are the ones involved in chemical bonding.

* Fluorine (F) is in Group 17 (also known as Group VIIA) of the periodic table. This means it has 7 electrons in its outermost shell.

* Chlorine (Cl) is also in Group 17, so it also has 7 electrons in its outermost shell.