Here's why:

* Lithium (Li) is a metal in group 1 of the periodic table, meaning it has a +1 charge when it forms an ion (Li⁺).

* Sulfur (S) is a nonmetal in group 16, meaning it has a -2 charge when it forms an ion (S²⁻).

To balance the charges and form a neutral compound, we need two lithium ions (2 x +1 = +2) for every one sulfide ion (-2). This results in the formula Li₂S.