Wrote «chemformula» section

004/codex-004.tex

@@ -506,7 +506,7 @@ As it turns out, people have written a number of different packages over the yea
 
 \section{Package \texttt{mhchem}}
 
-This package is useful for typesetting chemical equations and reactions and has a fairly intuitive interface. For example, to typeset this equation:
+The \texttt{mhchem} package is useful for typesetting chemical equations and reactions and has a fairly intuitive interface, making use of a \texttt{ce} environment. For example, to typeset this equation:
 
 \vspace{\baselineskip}
 \noindent{}\ce{CO2 + C -> 2 CO}
@@ -553,15 +553,46 @@ which will give us:
 
 \section{Package \texttt{chemformula}}
 
-For typesetting chemical compounds and reactions:
+the \texttt{chemformula} package is similar to \texttt{mhchem} in many respects, but is stricter about how certain items are input. In return, it has more options to customize the output.
 
-\ch{KCr(SO4)2 * 12 H2O}
+Again, it's pretty intuitive. To write the chemical formula for copper(II) sulfate pentahydrate, we would use code like this:
 
-\ch{[Cu(NH3)4]^2+}
+\begin{Verbatim}[]
+\ch{CuSO4 * 5 H20}
+\end{Verbatim}
 
-\ch{$2n$ Na + $n$ Cl2 -> $2n$ NaCl} \par
+which produces
+
+\vspace{\baselineskip}
+\noindent{}\ch{CuSO4 * 5 H2O}
+\vspace{\baselineskip}
+
+The most notable difference between \texttt{chemformula} and \texttt{mhchem} is that \texttt{chemformula} can distinguish between different types of input, which are separated by a space. That means that in our example above, there are four parts: the copper sulfate part, the asterisk part (which \texttt{chemformula} renders as a dot), the ``5'' part, and the H2O part. \texttt{chemformula} can then detect whether each input is a formula, a stoichiometric factor, an arrow, etc.
+
+You can also use math mode in \texttt{chemformula}. For example, this code:
+
+\begin{Verbatim}[]
+\ch{$2n$ Na + $n$ Cl2 -> $2n$ NaCl}
+\end{Verbatim}
+
+will give us this reaction:
+
+\vspace{\baselineskip}
+\noindent{}\ch{$2n$ Na + $n$ Cl2 -> $2n$ NaCl}
+\vspace{\baselineskip}
+
+Again, there are lots of options to customize the output. Here's one with fractions:
+
+\begin{Verbatim}[]
+\ch{3/2}
+\ch[frac-style=xfrac]{3/2} 
+\end{Verbatim}
+
+\vspace{\baselineskip}
+\noindent{}\ch{3/2}  \par
+\noindent{}\ch[frac-style=xfrac]{3/2} 
+\vspace{\baselineskip}
 
-\ch{\{[CH2=CH-CH2]- <-> {}[CH2-CH=CH2]- \}} \par
 
 \section{Package \texttt{chemfig}}