Here's why:

* Calcium (Ca) is a Group 2 element, forming a +2 cation (Ca²⁺).

* Hydrogen sulfide (H₂S) is a weak acid. When it acts as an acid, it loses a proton (H⁺) and forms the hydrosulfide ion (HS⁻).

* To balance the charges, two hydrosulfide ions (HS⁻) are needed for each calcium ion (Ca²⁺).

Therefore, the formula is Ca(HS)₂.