Step 1: Understand the definition of a compound. A compound is a substance formed when two or more different elements chemically bond together in fixed proportions.
Step 2: Analyze each option to determine if it fits the definition of a compound:
- CO (carbon monoxide) consists of carbon and oxygen atoms bonded together, so it is a compound.
- O (oxygen atom) is a single element, not bonded to any other element, so it is not a compound.
- CO2 (carbon dioxide) consists of carbon and oxygen atoms bonded together, so it is a compound.
- O2 (oxygen molecule) consists of two oxygen atoms bonded together, but since both atoms are the same element, it is a molecule of an element, not a compound.