Here's why:
* Fluorine's electron configuration: Fluorine has 9 electrons, with the configuration 2, 7. This means it has 7 electrons in its outer shell (valence shell).
* Octet rule: Atoms tend to gain or lose electrons to achieve a stable configuration with 8 electrons in their outer shell. This is known as the octet rule.
* Gaining an electron: Fluorine can achieve a stable octet by gaining one electron, resulting in a negative charge (-1). This creates the fluoride ion (F-).
Therefore, fluorine gains one electron to become a fluoride ion.
