The sulfur atom in H₂SO₃ has sp³ hybridization.
To determine the hybridization of S, you must:
a. Draw the Lewis structure for H₂SO₃.
b. Determine the steric number (SN) of the S atom.
SN = BP + LP, where
BP = number of bonding pairs and
LP = number of lone pairs.
c. Assign the hybridization that corresponds to the SN.
The Lewis structure of H₂SO₃ is
There are three bonding pairs and one lone pair on the S atom, so SN = 4.
The hybridization that corresponds to various steric numbers is
SN = 2; Hybridization = sp
SN = 3; Hybridization = sp²
SN = 4; Hybridization = sp³
SN = 5; Hybridization = sp³d
SN = 6; Hybridization = sp³d²
We see that SN = 4 corresponds to sp³ hybridization for the S atom.