When lithium and fluorine combine to form lithium fluoride, they form an ionic bond.

In simple terms, lithium (a metal) loses one electron to become a positively charged Li⁺ ion, and fluorine (a nonmetal) gains that electron to become a negatively charged F⁻ ion. The strong electrostatic attraction between these oppositely charged ions is what we call an ionic bond, giving the compound lithium fluoride (LiF).