# What happened to writing ions with \ce?

I took a look at some old questions and noticed that some \ce{} expressions were not working as they should. I am especially interested in how to write polyatomic ions. The old way results in numbers that are out of phase.

According to these pointers \ce{CrO4^2-} should work. $$\ce{CrO4^2-}$$

A similar guideline page gives \ce{ CrO4^2- } instead. $$\ce{ CrO4^2- }$$

Trying a few on my own,

$$\tag{\ce{CrO_4^{2-}}}\ce{CrO_4^{2-}}$$ $$\tag{\ce{{{{CrO}_4}^2}^{-}}}\ce{{{{CrO}_4}^2}^{-}}$$

I finally found a working result that does not invoke mhchem: \mathrm{CrO}_{4}^{2–}, or

$$\mathrm{CrO}_{4}^{2–}.$$

Why does this occur? What is the \ce way to write \mathrm{CrO}_{4}^{2–}?

• Related (concerning the opposite problem with an older version of the mhchem extension of MathJax): Formulae with charge and number of atoms (e.g. sulfate ion) aren't displayed properly
– user7951
Dec 23 '16 at 20:12
• In the android app the old formatting is still used, but we already know that the versions differ. Dec 23 '16 at 21:56
• All the chromate ions in the center of your post display correctly (and exactly the same) for me. Were they not displaying correctly earlier? or it appears this has been fixed now? Feb 27 '18 at 5:19

This change was introduced with MathJax/mhchem v3.

That's because it is the recommended way of writing.

• It is IUPAC's recommendation. Red Book section IR-4.3 shows examples, but is missing a clear text description. The Green Book states "The staggered arrangement is now recommended" and refers to the Red Book.

• It is ACS's recommendation. The ACS Style Guide says "Stagger the subscript and superscript; do not align them".

• This notation is used by a lot of authors and publishers nowadays, for instance Zumdahl.

I would have offered an option to switch between the different layout options, but I didn't see any usable way.

• What's your personal take on this matter? I would've probably just kept the old way and added something like \ce{CrO4\ ^2- for the IUPACees :) Dec 24 '16 at 20:23
• I have a long history of disliking most IUPAC decisions that I am familiar with, such as $\mathrm{d}x$ instead of $dx$ for differentials, their crazy mass spec terminology, etc., but on this issue I squarely agree with them. The offset superscript is (to me) much clearer, less confusing, and easier to read. Dec 26 '16 at 5:18

This was a change that was recently introduced to mhchem. (‘recently’ meaning ‘in the latest version’.) The change was made because IUPAC explicitly says the charge should not be above the index of the preceeding element. Which makes sense if you think about it in most cases (the charge of $2-$ belongs to $\ce{CrO4}$, not just to the $\ce{O}$) — but it is still weird for most of us.

• Really, IUPAC? Oh, boy. They recommend $$\ce{ CrO4^2-(aq)}$$ instead of the much more reasonable-looking $$\mathrm{CrO}_{4}^{2–}\mathrm{(aq)}\ \text{or}\ \mathrm{CrO}_{4\mathrm{(aq)}}^{2–}?$$ The rationale you suggest is probably the reason but everyone who understands what $\mathrm{CrO}_{4}^{2–}$ is should understand its (qualitative) charge distribution :=) shakes head Dec 24 '16 at 0:03
• @LinearChristmas I dunno, I’ll just silently point over to Loong ;)
– Jan
Dec 24 '16 at 0:06
• @Loong's answer in the link he provided agrees with your statement. Furthermore, Martin once noted on a question that $\ce{(aq)}$ is recommended not as a subscript. How one will write lengthy ionic equations using both rules remains a mystery. Dec 24 '16 at 0:12
• @LinearChristmas This may help with long equations on SE ;)
– Jan
Dec 24 '16 at 0:15
• @LinearChristmas IUPAC uses this new notation (I don't find any explicit recommendation), ACS recommends it. However, according to ISO, CEN, DIN, and basic principles of typography, this notation is wrong.
– user7951
Dec 24 '16 at 0:28
• Anway, what bothers me the most is the fact that the unexpected update of MathJax/mhchem retroactively changed old posts.
– user7951
Dec 24 '16 at 0:38
• @LinearChristmas I'm not sure whether "reasonable-looking" is as objective as your comment seems to suggest. I think staggering the subscript and superscript looks perfectly reasonable to me. Whether it is correct is of course another issue.
– orthocresol Mod
Dec 25 '16 at 13:58
• @orthocresol I see what you mean. Currently, I defined 'reasonable-looking' as in $(1)$ most applicable for long ionic equations, takes up less space, looks more compact; $(2)$ is more similar to how one writes using pen and paper. Dec 25 '16 at 14:02
• @LinearChristmas Haha, my pen and paper style is a complete hodgepodge: i.stack.imgur.com/oid62.jpg
– orthocresol Mod
Dec 25 '16 at 14:10
• @orthocresol I would say it is more aligned than it is staggered, mainly because this furthers my agenda. You will probably have a job waiting at IUPAC should you really conform to a staggered style in pen ;) Dec 25 '16 at 14:18
• $\ce{[CrO4]^2-}$ Jan 3 '17 at 6:51