Complete the statement.
Carbon dioxide is ().

 - a compound
 - an elementary substance
You can tell whether carbon dioxide is an elementary substance or a compound by counting the number of symbols in its chemical formula. A symbol contains either one capital letter or a capital letter followed by one or two lowercase letters.
The chemical formula for carbon dioxide is CO2. This formula contains two symbols: C for carbon and O for oxygen. So, the formula tells you that carbon dioxide is made of two chemical elements bonded together.
Substances made of two or more chemical elements bonded together are compounds. So, carbon dioxide is a compound.