That will give math italic letters.
Put the following in a chem.sty file and then load it with
\usepackage{chem}:
\DeclareRobustCommand{\chem}[1]{%
\mbox{%
\m@th
\if b\expandafter\@car\f@series\@nil\boldmath\fi
\ensuremath{\mathrm{#1}}%
}%
}
\endinput
Then try this test file. Switching to boldface is done
automatically.
\documentclass{article}
\usepackage{chem}
\begin{document}
\title{Title}
\author{A. U. Thor}
\maketitle
\tableofcontents
\section{Water is \chem{H_2O}}
Water is
