The correct formula unit for the ionic compound chromium(II) chloride is:
1 Cr²⁺ ion and 2 Cl⁻ ions
So, the answer is:
? 1 Cr²⁺ ion and 2 Cl⁻ ions
In this case, the chromium ion has a +2 charge (Cr²⁺) and each chloride ion has a -1 charge (Cl⁻). Therefore, it takes two chloride ions to balance the charge of one chromium ion, resulting in the formula CrCl₂.