Carbon monoxide. Not carbon monooxide — the doubled vowel gets collapsed and everyone writes and says it with one o.
But the same collision happens in diiodine, with two i's in a row, and there the vowels stay. Nobody writes diodine. Same structural situation, opposite treatment.
Is there a rule that distinguishes them, or is one of these simply frozen usage that the other never acquired?
There is a rule, and the short version is that the elision is specific to mono-, not general.
The naming conventions permit dropping the final vowel of a prefix when it meets the vowel starting the element name, but in practice this is applied to mono- before an o, and essentially nowhere else. So monoxide is standard, and so is the pattern in tetroxide, where the a of tetra- disappears.
Di- has no vowel to spare in the same way. Its i is the entire prefix apart from the d, so dropping it destroys the word rather than smoothing it. That is the structural difference between the two cases, and it is why no rule was ever written that would produce "diodine".
The other half of the answer is frequency. Carbon monoxide is a substance ordinary people talk about, so its name got worn smooth by use. Diiodine is a term almost nobody says out loud, so nothing wore it down.
There is a further wrinkle worth knowing: di- and bi- do not mean the same thing, despite both suggesting two, and confusing them causes real errors.
In the older naming that survives on shop shelves, bicarbonate does not mean two carbonates. That prefix carries a historical meaning about acid salts. Meanwhile dichloride does mean two chlorines.
So the prefixes in chemical names are not a single consistent counting system. They are several systems from different eras layered on top of each other, and the vowel question you asked is one of the more harmless places where the layers show.
The practical rule I give students: write the full, unelided form unless the elided one is the standard name of a substance they will meet on a label.
That is roughly monoxide, dioxide and tetroxide, and almost nothing else. Everything outside that short list, spell it out and you will be right.
@bench_chem_bea · 3w ago
Worth adding that both spellings of the first one genuinely exist in the literature — you will find monooxide written out in full — and that current recommendations lean towards not eliding as the default, precisely because unpredictable elision makes names harder to parse and harder to search.
The direction of travel in chemical naming for about fifty years has been away from euphony and towards mechanical regularity. A name that can be generated by a rule can be generated by software, and a name that can be generated by software can be looked up.
Monoxide survives that pressure only because it is entrenched in everyday language, safety labelling and legislation. It is a fossil protected by how many places it is already printed.
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