In the contact process, SO3 is dissolved in concentrated Sulphuric acid to produce:
H2S2O7
H2SO4(I)+SO3(g)→H2S2O7(I)
Oleum, the product on the right hand side is also knows as fuming sulphuric acid. It has other names – pyrosulphuric acid or disulphuric acid.
It has the following structure:
Each Sulphur atom is sp3 hybridized as per VSEPR theory. Also, Sulphur retains its +6 oxidation much like in the case of the sulphate ion, Sulphuric acid and SO3.