why the lone  pair of electrons on sulphur in so2 is in an sp3 hybrid orbital?

The lone pair of electrons on sulphur in SO₂ is not in sp³ hybrid orbital but in sp² hybrid orbital. The electronic configuration of sulphur is [Ne]3s²3p⁴. One p-orbital is occupied by an electron pair while the remaining two contain one unpaired electron each. One electron of the p-orbital containing two electrons is promoted to 3-d orbital. Now all the three p-orbitals are singly occupied. The 3s orbital and two 3p orbitals hybridise to form three sp² hybrid orbitals. One of these orbitals contain a lone pair, while the remaining two contain one electron each. These overlap with the oxygen p orbitals and thus form sigma bonds. The unhybridised p and d orbital on sulphur participate in pi-bonding.