# Aligning reaction equation with amsmath

Basically, I'm trying to use amsmath to align the 2 possible outcomes of a reaction, which naturally does not include an equals sign.

$$\ce{^{2}_{1}H} + \ce{^{2}_{1}H} &\rightarrow \ce{^{1}_{1}H} + \ce{^{3}_{1}H} + 4.03 \text{MeV}\\ &\rightarrow \ce{^{1}_{0}n} + \ce{^{3}_{2}He} + 3.27 \text{MeV}\\$$


The reaction has a branch, 2 possible outcomes occur and I'm trying to align the arrows. Any advice?

• replace equation by align*. – Sigur Feb 12 '19 at 22:47
• ... and consider using siunitx for typesetting MeV. – user121799 Feb 12 '19 at 22:50
• @P227 I will substitute \mathrm{MeV} instead of \text{MeV} if you not will use siunitx. – Sebastiano Feb 12 '19 at 23:06

You could use split or a fancier typesetting.

\documentclass{article}
\usepackage{amsmath,siunitx}
\usepackage[version=4]{mhchem}

\DeclareSIUnit{\eV}{eV}

\begin{document}

$$\begin{split} \ce{^{2}_{1}H} + \ce{^{2}_{1}H} &\rightarrow \ce{^{1}_{1}H} + \ce{^{3}_{1}H} + \SI{4.03}{\mega\eV} \\ &\rightarrow \ce{^{1}_{0}n} + \ce{^{3}_{2}He} + \SI{3.27}{\mega\eV} \end{split}$$

$$\ce{^{2}_{1}H} + \ce{^{2}_{1}H} \mathrel{\begin{array}{@{}c@{}}\nearrow\\\searrow\end{array}} \begin{array}{@{}l@{}} \ce{^{1}_{1}H} + \ce{^{3}_{1}H} + \SI{4.03}{\mega\eV} \\[2ex] \ce{^{1}_{0}n} + \ce{^{3}_{2}He} + \SI{3.27}{\mega\eV} \end{array}$$

\end{document}


Even fancier, with tikz-cd:

\documentclass{article}
\usepackage{amsmath,siunitx}
\usepackage[version=4]{mhchem}
\usepackage{tikz-cd}

\DeclareSIUnit{\eV}{eV}

\begin{document}

$$\begin{tikzcd}[row sep=-1em,column sep=3em] & \ce{^{1}_{1}H} + \ce{^{3}_{1}H} + \SI{4.03}{\mega\eV} \\ \ce{^{2}_{1}H} + \ce{^{2}_{1}H} \arrow[ur,start anchor=real east,end anchor=west] \arrow[dr,start anchor=real east,end anchor=real west] \\ & \ce{^{1}_{0}n} + \ce{^{3}_{2}He} + \SI{3.27}{\mega\eV} \end{tikzcd}$$

\end{document}