A detector is calibrated for carbon monoxide, whose molecules contain equal numbers of carbon and oxygen atoms. Which formula belongs on its label?

Explanation

CO shows one carbon atom and one oxygen atom because neither symbol carries a digit. CO2 contains twice as many oxygen atoms as carbon atoms and therefore represents carbon dioxide instead. The distractor C2O2 also shows a 1:1 ratio, but the established molecular formula of carbon monoxide is CO - C2O2 names a different composition.