[Tex/LaTex] How to get the the dot symbol in Chemistry

symbols

How to get this dot in the picture?

enter image description here

Best Answer

R. Schumacher already answered what the symbol means. How to get it with LaTeX is a different question. Here are five different versions all of which don't look perfect in the default settings:

\documentclass{article}
\usepackage{chemformula,mhchem,array}
\newcommand*\cs[1]{\texttt{\textbackslash#1}}

\begin{document}

\newcounter{version}
\setlength\extrarowheight{2pt}

\begin{tabular}{>{\stepcounter{version}\theversion.\quad}ll}
  .           & $\mathrm{O}_2^{.-}$ \\
  \cs{cdot}   & $\mathrm{O}_2^{\cdot-}$ \\
  \cs{bullet} & $\mathrm{O}_2^{\bullet-}$ \\
  mhchem      & \ce{O2^{.-}} \\
  chemformula & \ch{O2^{.-}} \\
\end{tabular}

\end{document}

enter image description here

To me version 3. clearly is much too big to look good and not an option. Versions 1. and 2. are arguably too small although \cdot doesn't look too bad in my opinion. In the mhchem version the dot and the minus sign are very close while in the chemformula version they're not perfectly aligned. I prefer the chemformula version nonetheless because it is adjustable:

\documentclass{article}
\usepackage{chemformula}

\begin{document}

\ch{O2^{.-}}

\setchemformula{radical-radius=1pt}
\ch{O2^{.-}}

\setchemformula{
  radical-radius=.5pt ,
  radical-vshift=.444ex
}
\ch{O2^{.-}}

\end{document}

enter image description here

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