Final answer:
The formula unit for the compound formed by Ca2+ and P3- ions is Ca3P2, achieved by balancing the charges of the ions to create a neutral compound.
Step-by-step explanation:
The formula unit for the compound formed when Ca2+ and P3- ions interact can be determined by ensuring the total positive and negative charges in the compound balance out to zero. Since the calcium ion has a 2+ charge and the phosphorus ion has a 3- charge, we need to find the lowest common multiple of these charges, which is 6. Thus, we need 3 calcium ions (3 × 2+ = 6+) to balance 2 phosphorus ions (2 × 3- = 6-), resulting in the neutral compound Ca3P2. This method of creating neutral compounds is generally applicable in predicting the formulas of ionic compounds containing both simple and polyatomic ions.