I read that the Hydrogen atoms with nuclear spin in same direction are called Ortho- Hydrogen whereas those in opposite nuclear spin are Para- Hydrogen. But is there any resemblance of such a nomenclature with Ortho- Meta- Para- of Benzene, cause for benzene, these denote the relative position of substituent. Or is it just named so?

  • $\begingroup$ Coincidental naming. $\endgroup$
    – Poutnik
    May 24 at 11:57
  • 3
    $\begingroup$ There are ortho- and paracompact spaces in math (and meta-, too). No relation. We people are lazy bunch, we use the same word over and over again until it looks like a dirty rag, and then some. $\endgroup$ May 24 at 13:27
  • 1
    $\begingroup$ And there is also ortho and meta phosphoric acid as yet different usage, describing different hydration levels of element oxoacids. $\endgroup$
    – Poutnik
    May 24 at 15:04
  • $\begingroup$ For "ortho-"; for "para-"; for "meta-". $\endgroup$
    – Nat
    May 25 at 0:34

1 Answer 1


There is no connection of ortho- para- organic nomenclature with the nuclear spin isomerism of hydrogen. However, that raises an interesting point, that which nuclear state should be called the ortho- vs. para- or what was the thought process among the early German discoverers?

[Subscription only access: Farkas, L. (1933). Über Para- und Orthowasserstoff. In: Ergebnisse der Exakten Naturwissenschaften, vol 12. Springer, Berlin, Heidelberg. https://doi.org/10.1007/BFb0111841]

"Die antiparallele Kernspinorientierung entspricht einem, "Kernspinsingulett" Zustand mit dem statistischen Gewicht I, die parallele Kernspinorientierung dem , Kerntriplett"-Zustand mit dem statistischen Gewicht 3."

Then a footnote reads on later pages:

"...Im allgemeinen werden immer die Zustände mit dem größeren statistischen Gewicht als Orthozustände bezeichnet."

which means that the antiparallel nuclear spin orientation corresponds to a nuclear singlet state with a statistical weight of 1, the parallel nuclear spin orientation corresponds to the nuclear triplet state with a statistical weight of 3. In general, the states with larger statistical weight are designated as ortho- states in nuclear spin isomers.

Perhaps this ortho- nomenclature is due to the other meaning of ortho which means, ‘right, correct, proper’, so the majority (=higher statistical weight) of the hydrogen's nuclear isomer is in the correct form/proper form, hence this is ortho- nuclear spin isomer of hydrogen.

  • $\begingroup$ o-together, p-opposite there might be a connection. $\endgroup$
    – jimchmst
    Jun 19 at 15:03
  • $\begingroup$ @jimchmst, Are you suggesting that those hardcore physicists were inspired by organic nomenclature, that is most likely not the case :-) So far, I could not find any. $\endgroup$
    – AChem
    Jun 19 at 15:05

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