Embed it in a TikZ picture to show it on MHB:
\begin{tikzpicture}
%preamble \usepackage{chemfig}
\node {
\chemname{\chemfig{
C(-OH)(-[2]C-OH)(-[6]C-OH)
}}{Glycerol}
};
\end{tikzpicture}
Click on the picture to see the LaTeX code.
I haven't tried to draw Lewis diagrams or symbols with it yet, but I expect that chemfig supports that as well.
Of course there are also other LaTeX packages to draw chemical structures.
Regular chemical formulas can be done with the usual MathJAX on MHB.
Or you can put it in an actual LaTeX document by including the mhchem package.
So we can write for instance:
Code:
\ce{O2 + 2H2 -> 2H2O}
Put it between dollars and it shows up like:
$$\ce{O2 + 2H2 -> 2H2O}$$