Eli5 how do you know when to put the “mono” at the beginning of the second element when naming compounds?

Question

Eli5 how do you know when to put the “mono” at the beginning of the second element when naming compounds?

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Answer 1

Effectively “only if it’s important enough to stress that there’s only one atom for some reason”.

So normally it’s assumed that there’s one atom if there’s an absence of a relevant prefix.

But, for example, carbon dioxide is just so ingrained in common language, and carbon monoxide is just dangerous enough, that specifying to avoid confusion seems to have been deemed necessary