[tex]\underbrace{\hbox{NO}_{2} + \text{4H}}_{\hbox{reactants}} \rightarrow \underbrace{\hbox{2H$_{2}$O} + \text{N}}_{\hbox{products}}[/tex]
The reactants are all the substances on the left-hand side of the reaction arrow.
The products are all the substances on the right-hand side of the reaction arrow.
The elements are the atoms of H and N.
