Here's why:

* Mercury (I) refers to the mercury ion with a +1 charge (Hg⁺). Since it is a polyatomic ion, it is written as Hg₂²⁺.

* Sulfide refers to the sulfur ion with a -2 charge (S²⁻).

To balance the charges, two mercury (I) ions are needed for every one sulfide ion. This gives the formula Hg₂S.