Simple formulae
For simple chemical notation, you can using the math modal or basic LaTeX. The steps below illustrate how to create more complex chemical notation.

More complex chemical notation
Go to your article's Data folder from the toolbar and add a file called header.tex with the following line in it:

\newcommand*\chem[1]{\ensuremath{\mathrm{#1}}}
Then, go back to the article view, insert a LaTeX snippet and type some chemical formulae, for example:
$\chem{H_2} + (1/2)\,\chem{O_2} = \chem{H_2O}$
will produce:

$${}^{4}_{12}\mathbf{C}^{5+}_{2}$$
will produce: