Here's why:

* Valence electrons: Fluorine has 7 valence electrons (electrons in its outermost shell).

* Octet rule: Atoms tend to gain, lose, or share electrons to achieve a stable configuration with 8 valence electrons (the octet rule).

* Bonding: Fluorine needs only one more electron to complete its octet. It achieves this by forming a single covalent bond with another atom, sharing one electron.

Therefore, fluorine typically forms only one bond, making it a highly reactive element.