The chemical formula for chromium (III) and selenium is Cr₂Se₃.

Here's how to determine that:

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

* Selenium is in group 16 of the periodic table, so it typically has a -2 charge (Se²⁻).

To balance the charges, you need two chromium ions (2 x +3 = +6) and three selenium ions (3 x -2 = -6). This gives you the formula Cr₂Se₃.