Final answer:
The formula unit for a compound made from Ca and O is CaO, representing a 1:1 ratio of calcium to oxygen atoms to form a neutral compound.
Step-by-step explanation:
The formula unit for a compound made from calcium (Ca) and oxygen (O) is CaO. Calcium typically forms a 2+ charge (Ca2+), and oxygen typically forms a 2- charge (O2-). Since each Ca2+ can balance with one O2-, the resulting compound is electrically neutral, which is why the formula unit CaO represents a ratio of one calcium atom to one oxygen atom.