The hybridisation of sulfur in anion SO42- is given to be sp3. Why is it so? Sulfur has two double bond and two single bond with four oxygen atoms, meaning there are a total of 12 electrons surrounding sulfur. sp3 only accomodates up to 8 electrons. besides, sp3 means that we are assuming there are only four single S-O bonds, which is not the case since resonance structure for it exists.

Also given that we know resonance structure exists for the sulfate ion, it must mean there is an unhypridised p orbital on sulfur. if sulfur were to be sp3 hybridised, there will not be any unhybridised p orbital to overlap with other p orbitals of oxygen. shouldn't the hybridisation of sulfur have beeen spd2? two unhybridised p orbitals, as well as four spd2 orbitals, so there will be 4 hybridised orbital to overlap with the ones in oxygen, and 2 overlap of p orbitals to form two double bonds?

Edit: in order for resonance structure for sulfate ion exists, does it mean that one sulfur atom would need to have 4 unhybridised p orbitals to overlap with each p orbitals from 4 oxygen atom? or would 1 unhybridised p orbital for sulfur will do? [likewise for CO32-, NO3-]

As the electron geometry about the S atom in SO4 2- is tetrahedral (in order to maximize stabilities by minimizing electron pair repulsions), hence sp3 hybridized orbitals are required for the sigma bonding about the S atom.

At A levels, the usual major resonance contributor taught to students, is that of an expanded octet with no separation of formal charges (at least in part because formal charges are not usually taught to most H2 students in the first place), ie. Fig 1 below.

But experimental evidence has indicated that such a resonance contributor, is actually only a minor resonance contributor. The major resonance contributor, is actually that of a dipositively formal charged S atom, singly bonded to 4 O atoms with uninegative formal charges, ie. Fig 2 below.

The (almost negligible) pi bonding that exists (for the minor resonance contributors), utilize the d orbitals of the S atom to overlap (poorly) with the p orbitals of the O atoms.

In other words, the bonding in the SO4 2- ion, is actually covalent with significant ionic character (due to the formal charges present in the major resonance contributor).

Experimental evidence has shown that the actual charge on the S atom is only slightly less than its formal dipositive charge, as such an S 2+ atom would accordingly be strongly electron-withdrawing by induction, withdrawing electron-density from the singly bonded O- atoms, towards the central S 2+ atom.

Concordantly, consequently and most interestingly, you could say that the (slight) partial double bond character (ie. in terms of bond length and bond strength) of the S=O bonds in the SO4 2- ion, is partly (only slightly) due to resonance, and partly (somewhat moreso) due to induction.