# Empty boxes in mhchem environment

I'm writing a radioactive decay worksheet for my students, and I want them to be able to fill in radioactive decay equations themselves.

I've tried to make it so that there are some empty boxes for the students to write the proton number, mass number, and atomic symbol by themselves.

I've succeeded in getting suitably formatted empty boxes for the proton and mass number using the following hack:

\ce{^{227}_{90}Th -> ^{4}_{2} \alpha {+} $_{\fbox{}}^{\fbox{}} \framebox[1cm][s]{ }$}


but the atomic symbol is badly formatted.

Any ideas on how to fix this?

I've also tried a tikz solution which works somewhat better, but it doesn't align nicely with the superscripts and subscripts:

\ce{^{227}_{90}Th -> ^{4}_{2} \alpha {+}} $\begin{tikzpicture}[scale=0.2] \draw[] (0,0)rectangle(1,1) (0,1.2)rectangle(1,2.2) (1.2,0)rectangle(2.7,2.2); \end{tikzpicture}$


Let's first start, with correcting your \ce usage.

• The {+} will lead to wrong spaces. Use ␣+␣.
• Write \alpha{}␣ so that LaTeX does not eat up the space afterwards.
• Don't put a space before \alpha.
• You can leave away all those unnecessary braces.
• Your alpha should be set upright, so I inlcude the textgreek font (but you could use any other, see the mhchem manual).

Your TikZ environment can take the baseline option to shift it vertically.

Lastly, I think the boxes need to be larger than the printed text, if they are for students to fill in with a pen.

\documentclass{report}
\usepackage{textgreek}
\usepackage[version=4]{mhchem}
\usepackage{tikz}
\begin{document}
\newcommand\isotope{
\begin{tikzpicture}[scale=0.5, baseline=-4pt]
\draw (0,0.1) rectangle (1.5,1.1)
(0,-0.1) rectangle (1.5,-1.1)
(1.7,-0.7) rectangle (3.2,0.7);
\end{tikzpicture}}
\ce{^227_90Th -> ^4_2\alpha{} + \isotope}
\end{document}


The \fbox approach should work, but keep it in the context of the \ce argument using phantoms of the actual answers (not being a chemist, I did not use the actual answers). Changing \fboxsep can make the boxes slightly larger or smaller, as desired.

\documentclass{article}
\usepackage{mhchem}
\begin{document}
\fboxsep=0.5pt
\ce{^{227}_{90}Th -> ^{4}_{2} \alpha {+} ^{\fbox{\phantom{227}}}%
_{\fbox{\phantom{90}}}{\fbox{\phantom{Th}}}}
\end{document}