$H _{2} SO _{4}(l)+ SO _{3}( g ) \rightarrow H _{2} S _{2} O _{7}($ oleum $)$

The structure of oleum is shown below :

$\begin{array}{*{20}{c}}
  {\begin{array}{*{20}{c}}
  {O\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,O} \\ 
  {||\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,||} 
\end{array}} \\ 
  {H - O - S - O - S - O - H} \\ 
  {||\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,||} \\ 
  {O\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,O} 
\end{array}$