Here's why:

* Octet rule: Atoms tend to gain, lose, or share electrons to achieve a stable configuration of eight electrons in their outermost shell (valence shell).

* Fluorine's electron configuration: Fluorine has 9 electrons. Its electronic configuration is 2, 7. This means it has 7 electrons in its valence shell.

* Gaining an electron: By gaining one electron, fluorine would have a full outer shell with 8 electrons, fulfilling the octet rule.