chemfig: lewis structure

Question

Currently working on corrosion of CaCO3:

\documentclass[a4paper]{article}
\usepackage{chemfig}

\begin{document}

\[
[Ca^{2+};
\chemfig{O - C (=[6]O) (-[8]O) }] + 2 \chemfig{H-Cl|}   \hspace{4em}|\Delta H<0
\]
\end{document}

But I need some help to fix some issues:

• Where did the Cl in HCl go?
• How do I draw the electron pairs around the O-atoms (see picture below)?
• Can I vertically center the lewis structure of CO3 inside the formula somehow?

Answer 1

\documentclass{article}
\usepackage{chemfig}

\begin{document}

\schemestart
\chemleft[
  \subscheme{
    \chemfig{Ca^{2+}} \arrow{0}[,0] \hspace{5mm}
    \chemfig{{\chemabove[-0.5pt]{\lewis{246,O}}{\scriptstyle\hspace{-7mm}\ominus}}- C (=[6]O) -[8]{\chemabove[-0.5pt]{\lewis{026,O}}{\scriptstyle\hspace{7mm}\ominus}}{}}}
  \hspace{5mm}
\chemright]
+2 \chemfig{HCl}
\hspace{4em}
$|\Delta H<0$
\schemestop

\end{document}

In the above example, I have made use of the \lewis command in order to add the free electron pairs to the oxygen atoms. For the circled charges, I have used \ominus as described in this answer and on page 42 (section 12.2 Add a superscript without modifying a bond) of the chemfig manual. In order to vertically center the formula, I have used an invisible \arrow as described in this answer. For the square brackets, I have used \chemleft[ and \chemright] as described on page 54 (8 Subscheme) of the chemfig manual.

Answer 2

To answer your questions :

• The Cl disappeared because the | has seemingly no sense for chemfig at least in that position, which probably leads to a non warned error.
• Drawing the electron pairs can be achieved using the \lewis command as pointed out by TeXnician. Please see section 8 p 30 of the chemfig manual

Please see the modified (although incomplete) example below.

\documentclass[a4paper]{article}
\usepackage{amsmath}
\usepackage{chemfig}

\begin{document}

\[
\begin{bmatrix}
Ca^{2+}; & 
\chemfig{
\lewis{2 4 6,\chemabove[0.8pt]{O}{\scriptstyle\hspace{-6mm}\ominus}}-
C (=[6]\lewis{7 5,O})-
\lewis{0 2 6,\chemabove[0.8pt]{O}{\scriptstyle\hspace{6mm}\ominus}}}\quad
\end{bmatrix} + 2 \chemfig{H-\lewis{0,Cl}}   \hspace{4em}|\Delta H<0
\]
\end{document}