答案1
\documentclass{article}
\usepackage{chemfig}
\begin{document}
\setchemfig{atom sep=1.5em, fixed length=true}
\setcharge{extra sep=3pt}
\schemestart
\chemfig{-\charge{90=\:}{N}=C=\charge{90=\:,270=\:}{O}}
\arrow{<->}
\chemfig{-\charge{90=\:}{N}=\charge{90=$ ^+ $}{C}-\charge{0=\:,90=\:,270=\:,45=$ \;^- $}{O}}
\arrow{<->}
\chemfig{-\charge{90=\:,270=\:,135=$ ^-\; $}{N}-\charge{90=$ ^+ $}{C}=\charge{0=\:,90=\:}{O}}
\schemestop
\end{document}