9

How to get this dot in the picture?

enter image description here

  • 3
    Is it \bullet...? – Najib Idrissi Feb 5 '14 at 20:02
  • Thank you for your answer! I made your comment answer and mentioned you so I remember this later too. – Léo Léopold Hertz 준영 Feb 5 '14 at 20:06
  • @cgnieder The symbol looks like this dl.dropboxusercontent.com/u/62073194/… in the first package. I think bullet is better. – Léo Léopold Hertz 준영 Feb 5 '14 at 22:55
  • @Masi: I did. You get no satisfying results first, but you can click on “Select from the complete list!” and then you get good results, among others of course. – Speravir Feb 5 '14 at 22:56
  • @cgnieder: But the question is asked in a wrong way IMHO. – Speravir Feb 5 '14 at 23:05
12

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

10

That symbol when used in Chemistry (usually with the full bonds between atoms) denotes an extra free electron (it negative charge). The minus is actually saying it twice.

These are usually used in Lewis Dot Structures (Diagrams) and mean a free electron (unbonded). Lewis Structure

Or when you see this you are describing an Oxygen radical (ie negative charge).

Also this entire area of notation is very much author and sub discipline dependent, so you need to verify exactly what the author/discipline in meaning in that context.

And as the preceding offered you would just use \bullet.

  • Very good addition! I have used convention that the bullet says if the species is radical and the number there says how much charged. – Léo Léopold Hertz 준영 Feb 5 '14 at 20:19
  • 1
    I have to disagree that "The minus is actually saying it twice", and it's misleading to say that "an extra free electron" is synonymous with "a negative charge". While there's arguably a degree of tautology (some might say 'precision') in writing $\ce{O2^{.-}}$ -- as an $\ce{O2}$ radical will always have a negative charge, and a negatively charged $\ce{O2}$ will always be a radical -- the two things are by no means synonymous. In other species, for instance atomic oxygen, a lone oxygen atom, $\ce{O^{.}}$ has no charge, while an oxygen ion, $\ce{O-}$ is not a radical. – owjburnham Sep 20 '17 at 14:22
  • (Please excuse mhchem notation in the above comment. I forgot that I wasn't on Chemistry StackExchange.) – owjburnham Sep 20 '17 at 14:25

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