Simple chemical notation
For simple chemical notation, you can using the via the x^2y button in the toolbar. All details here.

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: