Numerical Prefixes for Molecular Compound Names
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Naming a molecular compound's atom counts uses Greek numerical prefixes on each element, with mono- usually dropped from the first element only.
CO2 is carbon dioxide (mono- dropped from carbon since it appears first, di- kept on oxygen) while CO is carbon monoxide (mono- kept on the second element). N2O5 is dinitrogen pentoxide, keeping di- on nitrogen since it is not the first element being modified. P4O10 is tetraphosphorus decaoxide, both prefixes retained since neither element is limited to one atom. SO3 is sulfur trioxide, mono- dropped from sulfur as the first element while tri- marks oxygen's three atoms.
Dropping mono- from the second element instead of the first, or dropping it everywhere out of habit, turns CO into an ambiguous "carbon oxide" that no longer distinguishes it from CO2.