The formula for chromium(II) iodide is CrI2, and the formula for chromium(III) iodide is CrI3. As denoted by the Roman numerals in the parentheses, chromium(II) carries a charge of +2, while chromium(III) carries a charge of +3.
The number of iodine atoms differs depending on the chromium ion because the charges in the equation must balance out. Iodine carries a charge of -1, so chromium(II) requires two iodine atoms to balance out its charge. Chromium(III) requires three iodine atoms to neutralize its +3 charge. To determine the formula for chromium iodide, the charge of the chromium ion must be known.