As of recent, I provided an answer to the question Law of mass action & collision theory. Therein I wrote a generic reaction

$$\ce{$r_1$R1 + $r_2$R2 + \ldots + $r_n$R_n <=> $p_1$P1 + $p_2$P2 + \ldots + $p_m$P_m}.\tag{1}$$

It seemed natural to write stoichiometric coefficents in Italic, and use upright Roman for the species themselves.

  • Italic would be in line with the fact that the coefficients are numbers, and vary with reactions (are different for different transitions).
  • Upright Roman is how we usually write chemical species, e.g., $\ce{NaCl}$.

Though if I were to overthink, the $\ce{R_$i$}$ would also seem to be variables as such. Since this is a generic reaction, $\ce{R_$i$} \in \{\mathrm{every\ imaginable \ chemical \ species}\}$. Using this logic, I ought to write $(1)$ instead as

$$r_1R_1 + r_2R_2 + \ldots + r_nR_n \ce{<=>} p_1P_1 + p_2P_2 + \ldots + p_mP_m.\tag{1'}$$

  • Is there an official example or guideline how to handle this situation?

Personally I wholeheartedly support option $(1)$ but is this what is recommended?

  • What about the subindices? Upright or Italic?

Note that this qualitatively differs from Proper typography of “$\ce{NO_x}$”. There the number of elements actually changes within a chemical species (marks the number of carbons in alkanes for example) whereas here it is more for labelling purposes.

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    $\begingroup$ Since Chem.SE is going through $18$th-turn-$19$th century Vienna in terms of typography expertise, this is probably the best time to ask such a question. $\endgroup$ Feb 22, 2017 at 15:19
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    $\begingroup$ I'd agree with "It seemed natural to write stoichiometric coefficents in Italic, and use upright Roman for the species themselves." Not sure if ACS style guide would agree or not. $\endgroup$
    – MaxW
    Feb 22, 2017 at 16:01

1 Answer 1


Going by the basic rule of a variable (that can hold multiple values) being italic, I would go for the $r_nR_n$ style (4 times italics), but then I found the following in the IUPAC Green Book (printed page 53):

$$0 = \sum_j \nu_j \mathrm{B}_j \quad \text{where }\mathrm{B}_j\text{ denotes a species in the reaction ...}$$

Here, the B is upright although it is definitely a placeholder. I don't understand, why.

For the subindices, I think (hope) there is no doubt that the $n$ and $m$ should be italic, because they clearly are variables standing for numbers (either any number as in the sum, or for an unknown maximum number $1, 2, 3, \ldots, n$ as in the generic reaction). Both form are well established in mathematics (and I hope chemists don't deviate).

The IUPAC Gold Book, R05156 uses the same convention.

For the general chemical reaction : $$a\,\mathrm{A} + b\,\mathrm{B} \rightarrow p\,\mathrm{P} + q\,\mathrm{Q} + \dots% not using \ce to have proper formating regardless of mhchem version$$

And they do not mean Boron and Phosphorus.

(Spotted via this answer).

  • $\begingroup$ Great find! Luckily it is a recommendation I prefer, so that's nice. (regardless of whether it follows the variable/label rule) $\endgroup$ Feb 22, 2017 at 16:37
  • $\begingroup$ Added 2nd source $\endgroup$
    – mhchem
    Feb 27, 2017 at 8:14
  • $\begingroup$ If it helps your understanding, simply regard those as contractions or abbreviations. For example when we write a simplified formula for acetic acid, we may write $\ce{AcOH}$. We might abbreviate an aryl-moiety simply with $\ce{Ar\bond{-}}$, or any moiety with $\ce{R\bond{-}}$. I think in this particular case it is better to use upright characters to signify that these are chemical species. But this is just me guessing here... $\endgroup$ Feb 27, 2017 at 10:35

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