Chemistry - Why "monoxide" but not "diodine"?

Solution 1:

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]:


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


  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).




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


  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

Solution 2:

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

Nothing seems unusual about it.

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.

Solution 3:

"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:

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  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

Solution 4:

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.