Here's why:

* Electron Configuration: Fluorine's electron configuration is 1s²2s²2p⁵. This means it has 2 electrons in its first shell (1s²) and 7 electrons in its second shell (2s²2p⁵).

* Valence Electrons: Valence electrons are the electrons in the outermost shell of an atom. In fluorine's case, the outermost shell is the second shell, which contains 7 electrons.