Final answer:
The correct formula for the compound made of Calcium (Ca+2) and Carbonate (CO3-2) ions is CaCO3, which reflects a 1:1:3 ratio of Ca to C to O atoms.
Step-by-step explanation:
The formula for the compound formed by Calcium (Ca+2) and Carbonate (CO3-2) is CaCO3. Calcium carbonate is a compound, not an element, because it consists of more than one type of atom bonded together. In the formula CaCO3, the ratio of Ca to C to O atoms is 1:1:3 respectively.