# Why “monoxide” but not “diodine”?

When naming compounds, sometimes when there are two vowels in a row the second is elided: this happens for example with "mono-oxide", which becomes "monoxide" instead.

Why is this not always applied, e.g. with "diiodine"? Why aren't the repeated i's removed to make it "diodine"?

• As I see it, there's two possible reasons: (1) It's a bit more awkward to say monooxide than diiodine, hence the dropping of the extra "o". You also see that in "monopsony", which combines "mono" with "opsony" (buyer). [My MacOS disctionary defines it as "a market situation in which there is only one buyer."] (2) More commonly-used words get modified, through usage, more often than than less-commonly used words, and monoxide is more common than diiodine (though that wouldn't explain monopsony!). – theorist Jan 15 at 2:53
• One more example: EDTA (ethylenediaminetetraacetic acid) – Nilay Ghosh Jan 15 at 3:46
• Note that the "o" in "mono" is often elided not only when double "o" results: e.g. monatomic. – Ruslan Jan 15 at 10:02
• We do have cyclooctane, though. – Oscar Lanzi Jan 15 at 10:42
• The fact that "mono" is disyllabic probably has something to do with it. We can analyze "monoxide" as "mono" being reduced to "mon". If we had "diodine", it would be hard to analyze that as "di" being reduced to "d"; prefixes don't get reduced to just a consonant. A vowel prefix can be pronounced by itself, but a consonant prefix can only alter the syllable of the main word, rather than being pronounced in its own right. – Acccumulation Jan 16 at 22:45

Both "monooxide" and "monoxide" are used in the literature, yet "monoxide" is being used more often (Google Books Ngram Viewer). Although this is an accepted elision, it is not the preferred one, and must not set a precedent for other cases when multiplicative prefix ends with the same vowel as the root word begins with, such as "diiodine".

According to the current version of Nomenclature of Inorganic Chemistry, IUPAC Recommendations 2005 [1, p. 31]:

IR-2.7 ELISIONS

In general, in compositional and additive nomenclature no elisions are made when using multiplicative preﬁxes.

Example:

1. tetraaqua (not tetraqua)
2. monooxygen (not monoxygen)
3. tetraarsenic hexaoxide

However, monoxide, rather than monooxide, is an allowed exception through general use.

Further, from section IR-5.2 Stoichiometric names of elements and binary compounds [1, p. 69]:

The multiplicative preﬁxes precede the names they multiply, and are joined directly to them without spaces or hyphens. The ﬁnal vowels of multiplicative prefixes should not be elided (although ‘monoxide’, rather than ‘monooxide’, is an allowed exception because of general usage).

[…]

Examples:

[…]

1. $$\ce{NO}$$ nitrogen oxide, or nitrogen monooxide, or nitrogen monoxide

### References

1. IUPAC. Nomenclature of Inorganic Chemistry, IUPAC Recommendations 2005 (the “Red Book”), 1st ed.; Connelly, N. G., Damhus, T., Hartshorn, R. M., Hutton, A. T., Eds.; RSC Publishing: Cambridge, UK, 2005. ISBN 978-0-85404-438-2. IUPAC website
• @M.Farooq I wonder how much of that comes down to linguists having heard of monoxides. – J.G. Jan 15 at 16:21
• @J.G. I think it’s actually a matter of phonetics. English generally does not like doubled vowels outside of certain specific cases (and almost all words that have a doubled vowel that is not either a diphthong or collapsed to a single vowel sound (like in aardvark) are specifically loan words), so ‘monooxide’ would end up either pronounced just like ‘monoxide’ or as if it were two words (or possibly as if the ‘oo’ were an ‘oa’). – Austin Hemmelgarn Jan 15 at 16:36
• @J.G., I did not mention the context, the OED (Oxford English Dictionary) is a monumental work describing collecting all the English words since 13th century. It think in print, it is 20 volumes (it is not the free internet version). You will find all common chemistry terms, ancient and recent. If monooxide is not listed, but monoxide is, then it means that the linguists did not accept monooxide as a valid spelling even though if it were used by some. – M. Farooq Jan 15 at 16:54
• @M.Farooq I know what the OED is, I went to Oxford. In any case it's a decsriptive dictionary, so it's not telling us monooxide is invalid, it's telling us whoever prepared it hasn't read enough chemistry papers to have encountered it. They certainly won't be able to put any of these terms in their IUPAC context. – J.G. Jan 15 at 17:16
• @J.G., I think we have to stick to IUPAC rather than OED as you and andselisk say. The problem with anonymous Q&A sites is that it this gives us no clue who were are talking to. It could be a student or very well experienced respected scientist. Good for you that you were trained at such a good place. I still respect the level of work done by the OED board even for scientific terminologies. There are only three languages in the world who have this level of thoroughness with their monumental dictionaries of English, German and Urdu. I am not aware of others, if they exist in > 20 volumes. – M. Farooq Jan 15 at 17:33

Although it seems like a question of English language phonetics, monoxide is not an exception but a general trend.

Mon(a)oxide (vowel "o" dropped),

Dioxide (no "a" vowel in the prefix)

Trioxide (no "a" vowel in the prefix)

Tetr(a)oxide (vowel a dropped)

Pent(a)oxide (vowel a dropped).

The accepted spelling of diiodide is di-iodide in the grand Oxford Dictionary. It is also consistent with the other halides

difluoride, dichloride, dibromide, and di-iodide

As to the English language, someone wrote why b-u-t is pronounced differently from p-u-t? Some times we have to accept things as they are.

• On the contrary, the fact that this dropping of the vowel is not uncommon is all the more reason to consider it a linguistics question. And as to your "Some times [sic] we have to accept things as they are" advice, I disagree. Questions in linguistics are just as valid to pursue as questions in chemistry. What you said is like someone saying, in response to the question "Why is the sky blue?" that "sometimes we have to accept things as they are", to which any chemist or physicist would respond -- "hang on a sec, there is an explanation for that." – theorist Jan 15 at 4:38
• Well, that's a bit of a straw man. I wasn't saying this should get a funded study! I was simply saying it's a question worth asking on a linguistics forum. Just because we, as non-linguists, don't have an rigorous explanation, doesn't mean the question should be dismissed as not worth pursing. It's not something you or I have the background to dismiss. It could have a fascinating explanation. We need to have enough respect for other fields to appreciate that, even though we might not understand the value in pursing a question, an expert in that field might know something we don't. – theorist Jan 15 at 5:12
• @M.Farooq No, a beaker does not have a beak. The word is derived from ancient Greek βῖκος, a jug or wine jar. Calling your logic a "straw man" is insulting to straw, IMO. – alephzero Jan 15 at 15:47
• @alephzero, IMO, your "IMO" does not count! I was talking about the beaker in jest! Sorry that you missed it. – M. Farooq Jan 15 at 15:53

"Monooxide" was previously used to describe compound with single oxygen before being replaced with "Monoxide". Instance of "Monooxide" can be found in papers from 20th century which later became obsolete. Here is an example:

Reference:

1. Official Gazette of the United States Patent and Trademark Office: Patents, Volume 1055, Issue 2, U.S. Department of Commerce, Patent and Trademark Office, 1985

OED says for mono-: "before a vowel or h usually mon-." There's no such notation for di- (probably because a prefix that short can't afford to be reduced by 50%!). So there isn't a general rule in English or in chemistry that demands that identical vowels coalesce, but there is one that says that "mono-" is liable to drop its final "o" before any vowel, as in monarch, monaural, or monocle. This doesn't always happen (and it appears to be less common in modern scientific contexts), but it's still a regular feature.