Lithium (Li) and fluorine (F) form a single compound: Lithium fluoride (LiF).

Here's why:

* Lithium is an alkali metal, meaning it has a +1 charge when it forms an ion (Li⁺).

* Fluorine is a halogen, meaning it has a -1 charge when it forms an ion (F⁻).

* To form a neutral compound, the charges must balance. Therefore, one lithium ion combines with one fluoride ion to form LiF.