Here's why:

* Valence Electrons: These are the electrons in the outermost energy level of an atom. They're the ones involved in chemical bonding.

* Full Valence: Atoms are most stable when they have a full outer shell of electrons. For elements in the first two periods (like fluorine), a full valence shell means 8 electrons.

* Fluorine's Electron Configuration: Fluorine has 9 electrons. Its electron configuration is 2, 7, meaning it has 7 valence electrons.

* Gaining an Electron: To achieve a full valence shell, fluorine needs to gain one more electron, bringing its total valence electrons to 8.

This is why fluorine is highly reactive and tends to form ionic bonds by gaining an electron from another atom.