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.