Final answer:
The central sulfur atom in SO3^2- has one lone pair, contributing to a tetrahedral electron pair geometry and a trigonal pyramidal molecular structure.
Step-by-step explanation:
The sulfur trioxide ion, represented as SO32-, has a total of 32 valence electrons to be placed in its Lewis structure. In the case of SO32-, there is one sulfur atom surrounded by three oxygen atoms. Each oxygen atom forms a double bond with sulfur, leaving 26 electrons which are divided to form 13 lone pairs. However, these 13 lone pairs need to be divided among the oxygen atoms, with each having three lone pairs. Therefore, the central sulfur atom has one lone pair.
This because the total of 32 valence electrons minus the 24 electrons used in bonding (eight from each double bond), leaves 8 electrons or four pairs, which allocated three pairs to the oxygen atoms give us one remaining for the sulfur atom. The sulfur atom's lone pair combined with the three double bonds creates four areas of electron density, which results in a tetrahedral electron pair geometry and a trigonal pyramidal molecular structure, due to the repulsion between bond pairs and lone pairs causing a distortion from the ideal tetrahedral shape.