Is there a convention for breaking chemical formulae across lines?
I remember a couple of books from 1970-80s, where long formulas were split with what would be a $\rm \LaTeX$ \cdot "$\cdot$" notation, I believe. This is an example from Pope, M. T.; Jeannin, Y.; Fournier, M. "Heteropoly and isopoly oxometalates" in Inorganic Chemistry Concepts; Springer: Berlin, 1983; Vol. 8.:



The reason for that, it seems, is that the book was printed in A5 format. Personally, I find an option for mhchem to be able to split formulas across pages very useful.