# Identify atoms in chemical equation

I am trying to represent an exchange reaction in N2 + N, where the reactant atomic N is swapped for one in N2. I'm using the mhchem package, and currently have the following:

\documentclass{article}
\usepackage[version=4]{mhchem}
\begin{document}

The exchange reaction is:

\begin{align}
\ce{ N^{(1)} \bond{1} N^{(2)} + N^{(3)}
-> N^{(1)} \bond{1} N^{(3)} + N^{(2)} }
\end{align}

\end{document}


I'm unhappy with this for several reasons.

• It's not immediately clear that the superscript numbers are atomic identifiers.
• The spacing between bonded atoms is about the same as the spacing between molecules (+ vs -). I think having more space between molecules or less space between bonded atoms would help.

Perhaps I just need to fiddle with the spacing, especially because the superscripts are adding space. What is the best way to do that inside an mhchem equation?

Alternatively, I could completely ditch the superscript atomic identifiers and use a more clear distinction. Obviously, I need to specify that the reactant atomic N is now a component of N2, but how? Any suggestions are welcome.

## Edit 1

Removing the parenthesis and the space between bonds, as suggested in the comments,

\documentclass{article}
\usepackage[version=4]{mhchem}
\begin{document}

New:
%
\begin{align*}
\ce{ N^{1}-N^{2} + N^{3}
-> N^{1}-N^{3} + N^{2} }
\end{align*}

Old:
%
\begin{align*}
\ce{ N^{(1)} \bond{1} N^{(2)} + N^{(3)}
-> N^{(1)} \bond{1} N^{(3)} + N^{(2)} }
\end{align*}

\end{document}


This is clearly better, but the spacing before the bond (due to the superscript) still looks too large. What is the best way to fiddle with this?

• What about using colors? \ce{ N \bond{1} N + \textcolor{green}{N} -> N \bond{1} \textcolor{green}{N} + N } with xcolor.sty loaded? – Patrick Happel Mar 13 '17 at 21:00
• Thanks for the suggestion, but I'd rather stick with black and white. It's for a journal paper and I'm not planning for anything to be in color. Perhaps some other way of picking out the initial atomic N? – Ross Mar 13 '17 at 21:03
• As an alternative, you could use a colored (gray or black) background: \ce{ N \bond{1} N + \colorbox{gray}{N} -> N \bond{1} \colorbox{gray}{N} + N }. If you want to get rid of the spacing, adjust \fboxsep. – Patrick Happel Mar 13 '17 at 21:10
• Don't leave spaces around the bonds: \ce{ N^{(1)}\bond{1}N^{(2)} + N^{(3)} -> N^{(1)}\bond{1}N^{(3)} + N^{(2)} }. Or simpler \ce{ N^{(1)}-N^{(2)} + N^{(3)} -> N^{(1)}-N^{(3)} + N^{(2)} }. Personally, I'd just use superscripted numbers without parentheses \ce{N^{1}-N^2 + N^3 -> N^{1}-N^3 + N^2}. It is not uncommon to mark different substituents this way (\ce{R^{1}-R^2}). – cgnieder Mar 13 '17 at 22:09
• @clemens I hadn't noticed the difference with and without the space around \bond{1}, thanks. Unfortunately, I've actually been using the \ce{N-N} notation in practice. I like the suggestion about no parenthesis. – Ross Mar 13 '17 at 22:13

First of all: don't leave spaces around the bonds:

\documentclass{article}
\usepackage[version=4]{mhchem}
\begin{document}

\ce{
N^{(1)} \bond{1} N^{(2)} + N^{(3)}
-> N^{(1)} \bond{1} N^{(3)} + N^{(2)}
}

\ce{
N^{(1)}\bond{1}N^{(2)} + N^{(3)}
-> N^{(1)}\bond{1}N^{(3)} + N^{(2)}
}

\end{document}


Instead of \bond{1} it also suffices to input -:

\documentclass{article}
\usepackage[version=4]{mhchem}
\begin{document}

\ce{
N^{(1)}\bond{1}N^{(2)} + N^{(3)}
-> N^{(1)}\bond{1}N^{(3)} + N^{(2)}
}

\ce{
N^{(1)}-N^{(2)} + N^{(3)}
-> N^{(1)}-N^{(3)} + N^{(2)}
}

\end{document}


Personally I'd also leave the parentheses away. It is not uncommon to mark different substituents with supersctipt numbers (cf. \ce{R^1}). In my opionion the message becomes even clearer without the parentheses:

\documentclass{article}
\usepackage[version=4]{mhchem}
\begin{document}

\ce{
N^{(1)}-N^{(2)} + N^{(3)}
-> N^{(1)}-N^{(3)} + N^{(2)}
}

\ce{
N^{1}-N^2 + N^3
-> N^{1}-N^3 + N^2
}

\end{document}


Of course colors also work (unless you're colorblind):

\documentclass{article}
\usepackage[version=4]{mhchem}
\usepackage{xcolor}
\begin{document}

\ce{
{\textcolor{red}{N}}-\textcolor{blue}{N} + \textcolor{green}{N}
-> {\textcolor{red}{N}}-\textcolor{green}{N} + \textcolor{blue}{N}
}

\end{document}


And just as a comparison using chemfig:

\documentclass{article}

\usepackage{chemfig}
\begin{document}

\schemestart
\chemfig{N^1-N^2} \+ \chemfig{N^3}
\arrow
\chemfig{N^1-N^3} \+ \chemfig{N^2}
\schemestop

\end{document}


Here the length of bond and arrow is easily customizable.