Updated «mhchem» section

004/codex-004.tex

@@ -506,13 +506,50 @@ As it turns out, people have written a number of different packages over the yea
 
 \section{Package \texttt{mhchem}}
 
-For typesetting chemical equations:
+This package is useful for typesetting chemical equations and reactions and has a fairly intuitive interface. For example, to typeset this equation:
 
+\vspace{\baselineskip}
+\noindent{}\ce{CO2 + C -> 2 CO}
+\vspace{\baselineskip}
+
+we would simply use this markup:
+
+\begin{Verbatim}[]
 \ce{CO2 + C -> 2 CO}
+\end{Verbatim}
 
+As you can see, numbers placed after a letter are automatically formatted as a subscript. To format them as a superscript, we just use a caret (\texttt{\^}) before the number. For example, \verb|\ce{CrO4^2-}| produces \ce{CrO4^2-}. Pretty nifty, huh?
+
+Note that the superscripts and subscripts are not stacked; this is the preferred method according to the IUPAC\footnote{International Union of Pure and Applied Chemistry} Green Book. But if you want them to be stacked, \texttt{mhchem} has this option:
+
+\begin{Verbatim}[]
+\mhchemoptions{layout=stacked}
+\end{Verbatim}
+
+\mhchemoptions{layout=stacked}
+
+which will give us stacked superscripts:
+
+\vspace{\baselineskip}
+\noindent{}\ce{CrO4^2-}
+\vspace{\baselineskip}
+
+\mhchemoptions{layout=staggered-flat}
+
+It also does a pretty nice job of rendering fractions. \verb|\ce{1/2H2O}| gives us \ce{1/2H2O}. That's not the preferred way according to IUPAC, however. To accomplish that, you should use \verb|\ce{(1/2)H2O}| which gives us \ce{(1/2)H2O}. 
+
+You can also do some fancier things. Here's another example from the manual:
+
+\begin{Verbatim}[]
 \ce{Hg^2+ ->[I-] HgI2 ->[I-] [Hg^{II}I4]^2-}
+\end{Verbatim}
+
+which will give us: 
+
+\vspace{\baselineskip}
+\noindent{}\ce{Hg^2+ ->[I-] HgI2 ->[I-] [Hg^{II}I4]^2-}
+\vspace{\baselineskip}
 
-\ce{1/2H2O}
 
 \section{Package \texttt{chemformula}}