Here's how to understand this:

* Chromium is a transition metal with a variable charge, usually +3 in this case.

* Sulfur is a nonmetal with a charge of -2.

* To balance the charges, we need two chromium ions (2 x +3 = +6) and three sulfide ions (3 x -2 = -6).

Therefore, the formula for chromic sulfide is Cr₂S₃.