Here's how to understand it:

* Chromium(III) indicates that the chromium ion has a +3 charge (Cr³⁺).

* Oxide refers to the oxygen ion, which has a -2 charge (O²⁻).

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

Therefore, the formula is Cr₂O₃.