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](https://docs.moodle.org/27/en/Chemistry_notation_using_mhchem) `\ce{CrO4^2-}` should work.
$$\ce{CrO4^2-}$$

A similar [guideline page](https://docs.moodle.org/32/en/Chemistry_notation_using_mhchem#) 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–}`?