sulphate ion has tetrahedral geometry as predicted by VSEPR theory; the four S-O bonds are indistinguishable and have the same bond order, so the Lewis structure is better represented avoiding S=O double bonds; if instead of double bonds there are covalent dative bonds, what about the hybridization of S and O ? My idea is the following: Sulphur has hybridization sp3, each oxigen atom involved in dative bond has to provide an unfilled p orbital 
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Fabio
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Both structures cn be accepted. Lewis preferred the structure with 4 single bonds. L. Pauling preferred the mesomeric structure with two double bonds S=O – Maurice Jul 03 '20 at 09:33
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1Both structures might be accepted but the best way to draw it in the Lewis formalism is to put a double positive charge on sulphur and have all four oxygens be negatively charged. That also ensures complete equivalence of the oxygens without the need for resonance structures. – Jan Jul 03 '20 at 10:28
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See https://chemistry.stackexchange.com/a/1094/103633 and the Wikipedia page linked therein. – MichaelK Jan 20 '21 at 16:13