Eli5 how do you know when to put the “mono” at the beginning of the second element when naming compounds?
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When there’s only one of the second element, it’s necessary to distinguish between possible compounds with one of the second element and compounds with more than one of them, and there isn’t already a special name for the combination.
So we talk about carbon monoxide and carbon dioxide because both molecules are possible and the first has only a single oxygen. We don’t talk about dihydrogen monoxide because that compound is known as water.
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
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