Answer: The number of oxygen atoms
Explanation: Carbon Monoxide (CO) and Carbon Dioxide (CO2), are part of a group called Nonmetal Oxides. To name these kinds of compounds, you must always add a prefix to the last element named based on how many atoms it has. Because CO2 has 2 oxygen molecules, the prefix "di" is added. In Carbon Monoxide's case, there is only one oxygen molecule, so the prefix "mono" is added.