* Atomic Structure: Fluorine (F) is a non-metal located in Group 17 (also known as the halogens) of the periodic table. It has 9 protons and 9 electrons in its neutral state.

* Octet Rule: Atoms tend to gain, lose, or share electrons to achieve a stable configuration of 8 electrons in their outermost shell (valence shell). This is called the octet rule.

* Gaining an Electron: Fluorine, with 7 electrons in its valence shell, needs to gain one more electron to complete its octet. When it gains this electron, it becomes a fluoride ion (F-).

* Charge: Since the fluoride ion has gained one extra electron, it has one more negative charge than positive charges. This results in a net charge of -1.

In Summary: The fluoride ion gains a negative charge because it gains an electron to fulfill the octet rule and achieve stability.