Final answer:
In a molecule of CaCO3, there are 3 atoms of oxygen, 1 atom of calcium, and 1 atom of carbon. It is a compound with a Ca:C:O ratio of 1:1:3.
Step-by-step explanation:
In the formula CaCO3, there are 3 atoms of oxygen, 1 atom of calcium, and 1 atom of carbon. Calcium carbonate is a compound because it is made up of more than one element chemically combined in a fixed ratio. Specifically, the ratio of Ca : C : O atoms in the formula is 1:1:3. Each molecule consists of a single calcium ion with a +2 charge that balances perfectly with the -2 charge of a single carbonate ion (CO32-), which in turn is made up of one carbon atom and three oxygen atoms.