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"Mono" means singular, and "atomic" stands for the atom. So combining them will give a single atom - "monatomic".

But why is this so? Why can't it be "monoatomic"?

Meaning:

Consisting of one atom.

Searching gives me this:

Monoatomic (monatomic) source

In which monoatomic is the main word form. I could not find anything else online about this dual usage.

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    ... Alpha decay. Jul 2, 2022 at 11:09
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    @EdwinAshworth Isn't it omega decay? Jul 2, 2022 at 21:35
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    Related in chemistry.SE: Why "monoxide" but not "diodine"? Jul 3, 2022 at 9:13
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    @Acccumulation I think it's actually omicron, but that might be better avoided. Jul 3, 2022 at 14:55
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    Most English diphthongs are metastable at best having high energies of formation, small absorption cross sections, and short half-lifes.
    – Phil Sweet
    Jul 4, 2022 at 15:29

2 Answers 2

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OED says that monatomic is formed within English, by compounding; probably modelled on a French lexical item and provides the etymology below:

< mono- comb. form + atomic adj., probably after French monatomique (A. M. Gaudin 1833, in Ann. de chim. et de physique 52 115)

However, OED includes both forms:

1800s– monatomic, 1800s– monoatomic.

Today, in French, the form monoatomique is used. However, it appears that the earliest form used in French was monatomique and it was first used by A. M. Gaudin (a French chemist) in 1833. The French etymology dictionary from Centre national de ressources textuelles et lexicales (CNRTL) includes the same origin (as OED) but doesn't list the early form monatomique. However, you can find Gaudin's publications and the usage of monatomique in Google Books by searching "monatomique Gaudin". In English, monatomic was first used in 1848 (per OED) and it was based on French monatomique, the earlier form. Here is the earliest citation from OED:

Monatomic gases [Ger. einatomige Gase].
H. Watts tr. L. Gmelin Hand-bk. Chem. I. 53

The prefix mon- is an alternative form of mono- and OED mentions "before a vowel or h usually mon-." for the combining form mono-. I believe the formation with mon- is even more strict if the second element starts with the vowel 'o', like monoculus*. There isn't the form monooculus but OED includes the form monoculos (from 1500s) as well.

Wiktionary lists some other words prefixed with mon- (you can find more on OED):

  • monarchy

Note: Monarchy is an interesting case as the prefix mon- is not easily discernible and it is not a technical term. Monarchy is a borrowing, partly from French monarchie and partly from Latin monarchia and the ultimate origin is ancient Greek μοναρχία government by a single ruler ( < μονο- mono- comb. form + ἄρχειν to rule + -ία -y suffix.)[OED] The word monastery has a similar etymological path and it is from Hellenistic Greek μονάζειν to live alone ( < ancient Greek μόνος mono- comb. form + -άζειν suffix).[OED]

  • monaster

Note: The technical term monaster is very similar to monastery but has a different etymology, which is from mono- + aster (n.) (from Latin aster, < Greek ἀστήρ star).[OED]

  • monarthritic
  • monarthritis
  • monarticular
  • monaulic
  • monantherous
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    The most interesting example to me is monaural, which became popular (in contradistinction to binaural) in the early 1950s, but eventually lost out to monophonic (in contradistinction to stereophonic). I remember being mystified during the middle to late 1960s about why record covers were shortening "monaural" to "mono"—but, of course, they weren't.
    – Sven Yargs
    Jul 3, 2022 at 8:27
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    @SvenYargs Interesting indeed. Surprisingly, OED mentions that the term monaural is now preferred to monophonic in sound recording and reproduction. However, per Google Ngram, monophonic gained slight traction in recent years; despite monaural having an additional original sense ("Of or relating to the use of one ear only") as well.
    – ermanen
    Jul 3, 2022 at 10:14
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    "probably after French monatomique". Wait, what? In French, we use monoatomique. Jul 3, 2022 at 14:50
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    @EricDuminil It appears that the earliest form used in French was monatomique and it was used by Gaudin (a French chemist) in 1833. The french etymology dictionary CNTRL mentions the same origin as well but doesn't include the early form monatomique. However, you can find his books and the usage of monatomique in Google Books by searching "monatomique Gaudin". In English, monatomic was first used in 1848 (per OED) so it was based on French monatomique, the earlier form.
    – ermanen
    Jul 4, 2022 at 8:56
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It seems that "monoatomic" is the British spelling version and "monatomic" is American. It is said that it is rare to see "monoatomic". But both are correct.

According to Ichacha - Monoatomic:

The British spelling Monoatomic is just as correct as the American spelling Monatomic— Popsracer 13:02, 8 Mar 2004 (UTC)

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    This does not answer your question, about the reason behind the difference. If you don't adjust, I must CV the question as an easily found reference gives an answer with which you're satisfied. Jul 2, 2022 at 11:10
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    @EdwinAshworth That's not really how close-voting is supposed to work. NAA vote if you will, but close voting should be done in a Death of the Author sense. (If there's clarification in comments that makes a close-worthy question un-close-worthy, it should be edited into the question.) The presence or absence of an answer does not affect whether the question should be closed, even if that answer is by the OP.
    – wizzwizz4
    Jul 2, 2022 at 22:26
  • I'm pretty sure "monatomic" is the usual BrE spelling (source: native BrE speaker that once studied chemistry). "monoatomic" if anything is archaic.
    – abligh
    Jul 3, 2022 at 8:17
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    @wizzwizz4 From this answer, one can only deduce that the intended question was 'Are the variants "monatomic" and "monoatomic" both in use, and equally widespread?' Answering this requires only the reasonable use of general reference resources, for which CVing is recommended. // Here, ermanen (unlike DialFrost) answers the question as phrased, using a less easily available source. Hence, I have not CVd. Jul 3, 2022 at 15:03
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    @EdwinAshworth That's what the Not an Answer flag, or a delete vote, is for.
    – wizzwizz4
    Jul 3, 2022 at 15:11

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