documentclass{article}
usepackage{mhchem}

begin{document}

ce{{}^3He} has a higher vapour pressure, and therefore a lower boiling point,
than ce{{}^4He}: the result of this is that pumping away the most energetic
molecules from the bottom layer preferentially removes ce{{}^3He}.

end{document}

documentclass{article}
usepackage{mhchem}

newcommand{HeThree}{ce{{}^3He}}
newcommand{HeFour}{ce{{}^4He}}

begin{document}

HeThree has a higher vapour pressure, and therefore a lower boiling point, than HeFour: the result of this is that pumping away the most energetic molecules from the bottom layer preferentially removes HeThree.

end{document}

newcommand{HeThree}{ce{{}^3He}~}
newcommand{HeFour}{ce{{}^4He}~}