
\documentclass[12pt,oneside,a4paper]{article}

\usepackage[british]{babel}
\usepackage{amsmath}


\usepackage{mhchem}		% for \ce resp. chemical formula
\usepackage{chemfig}	% for showing a molecule

\begin{document}


	The chemical formula for water is H2O 		\\
	
	The chemical formula for water is $H_{2}O$		 \\

	The chemical formula for water is $\text{H}_{2}\text{O}$		 \\

	The chemical formula for water is H\textsubscript{2}O 		\\

	\newcommand{\tsb}{\textsubscript}	
	The chemical formula for water is H\tsb{2}O		\\ ~\\ ~\\



	% using mhchem package:
	The chemical formula for water is \ce{H2O} 		\\

        \chemfig{H-[:135]O-[:-135]H}             \\ ~\\



	\vskip 5em		% enforcing arbitrary vertical spacing

	

	The chemical formula of chromate is \ce{CrO4^2-}	\\ ~\\
		
	\ce{CH4 + 2 $\left( \ce{O2 + 79/21 N2} \right)$}	\\ ~\\ ~\\

	
	Exothermic reaction of sodium with water: \\

	\ce{ 2Na + 2 H2O -> 2Na_{(aq)}+ + 2OH_{(aq)}- + H2 } \\ ~\\
	% should the symbol for the minus charge drop then use: 2OH_{(aq)}^-


\end{document}
