Here's how to determine that:

* Potassium (K) is in group 1 of the periodic table, meaning it has a +1 charge.

* Sulfur (S) is in group 16 of the periodic table, meaning it has a -2 charge.

* To balance the charges, you need two potassium ions (2 x +1 = +2) to combine with one sulfide ion (-2).

Therefore, the formula is K₂S.