Since this has to do with the box alignment of sub/super script, I'm
fairly certain this has nothing to do with fontspec at all. Instead,
let's just write our own macro for isotopes:
\documentclass{minimal}
\usepackage{fontspec}
\setmainfont{Linux Libertine O}
% align by adding phantom text and moving the entire block left
\newcommand{\isotope}[3]{%
\newbox\aweight% atomic weight
\setbox\aweight=\hbox{#1}%
\newbox\anumber% atomic number
\setbox\anumber=\hbox{#2}%
% write isotope with phantom weight in number, and phantom number in weight,
% moved left by whichever is the least wide
{\ifdim \the\wd\aweight>\the\wd\anumber \hskip-\the\wd\anumber \fi%
\ifdim \the\wd\aweight<\the\wd\anumber \hskip-\the\wd\aweight \fi%
\ifdim \the\wd\aweight=\the\wd\anumber \hskip-\the\wd\anumber \fi%
\mbox{$^{\hphantom{#2}#1}_{\hphantom{#1}#2}$}${#3}$}}
% and of course we need a simple test text for this macro
\begin{document}
Let's look at our standard mildly radioactive \isotope{14}{6}{C}
isotope, and figure out where it goes. We could use something like
\isotope{228}{88}{Ra} but that would be overkill...
\end{document}
(there might be a more efficient way to do the dimension comparison, but
if there's one TeX topic that you're never going to find a normal,
simple explanation page for using google, it's working with plain box
dimensions and mathematical operations on dimensions for things like
if/then. It's all infuriatingly hidden in the TeX book and TeX by Topic,
and the minds of a handful of really good TeX/LaTeX/XeTeX package authors)
- Mike "Pomax" Kamermans
nihongoresources.com
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