Here's why:

* Lithium sulfide (Li₂S) is an ionic compound. This means it is formed by the electrostatic attraction between positively charged ions (cations) and negatively charged ions (anions).

* Lithium (Li) is a metal, and it tends to lose one electron to form a positive ion (Li⁺).

* Sulfur (S) is a nonmetal, and it tends to gain two electrons to form a negative ion (S²⁻).

* To achieve electrical neutrality in the compound, two lithium ions (2Li⁺) are needed to balance the charge of one sulfide ion (S²⁻).

Therefore, the negative ion in lithium sulfide is the sulfide ion (S²⁻).