Superscript in upper left to denote isotope in LaTeX

I'm not sure how nobody has posted mhchem, the canonical chemistry package! Straight from the manual, section "Isotopes":

\documentclass{article}

\usepackage{mhchem}

\pagestyle{empty}

\begin{document}

I have some \ce{^{227}_{90}Th+} lying around.

\end{document}

(\ce stands for "chemical equation" and can be used in either math or text mode.)

If you're doing other stuff with chemistry, this is the package you want. Check out the docs!

Smaller and lighter than mhchem is the isotope package.

Usage looks roughly like:

\documentclass{minimal}
\usepackage{isotope}

\begin{document}
\isotope{Po} --- \isotope[56]{Fe} --- \isotope[13][6]{C}\\

$\isotope{n} + \isotope{H} \to \isotope{D} +
\gamma(2.2\,\mathrm{MeV})$ \\

$\isotope[13]{C} + \alpha(5.314\,\mathrm{MeV}) \to
\isotope[16]{O}^{**} + n$
\end{document}

resulting in

You may wish to define a macro to typeset isotopes, say,

\newcommand\isotope[2]{\textsuperscript{#2}#1}

Use the macro as follows: \isotope{Si}{28}.

This approach avoids the use of math mode and associated low-level solutions such as ${}^{28}$Si.