Final answer:
The correct formula for the compound formed from calcium (Ca) and oxygen (O) is CaO, where each element forms ions with charges that balance out to create a neutral compound.
Step-by-step explanation:
The correct formula for the compound formed from Ca (calcium) and O (oxygen) is CaO. To determine this, we consider the charges on the calcium and oxygen ions. Calcium typically forms a Ca2+ ion, and oxygen typically forms an O2- ion. To balance the charges, one Ca2+ ion combines with one O2- ion to form CaO, with no net charge on the compound.