What is the Molecular Geometry of SO32-? 

The valence shell electron pair repulsion (VSEPR) theory of chemical bonding states that the ideal electron geometry or the actual molecular shape of a molecule or a molecular ion containing a total of 4 electron density regions around the central atom is tetrahedral. 

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However, the actual molecular geometry and shape of sulfite [SO3]2- ion differs from this ideal electron geometry owing to lone pair-bond pair electronic repulsions present in the molecule as well as a lone pair on the central sulfur atom. 

Hence, the molecular geometry or shape of SO32- is trigonal pyramidal while that of the SO3- ion is tetrahedral as per VSEPR. 

Lewis structure of SO32- 

The first step while drawing the Lewis structure of SO32- is to count the total valence electrons in each of the elemental atoms in the concerned molecule or ion. Moreover, one must check the stability of the molecule or ion before drawing it and minimize charges by converting lone pairs to bonds. 

Next, the molecular symmetry must be checked as per VSEPR. Hence, sulfite [SO3]2- has sp3 hybridization and the electronic configuration of its sulfur atom is 1s2 2s2 2p6 3s2 3p4. 

Sulfur is a VIA group atom while oxygen is at +4 oxidation state in the sulfite ion. Consequently, the total number of valence electrons in the molecule is 26. 

The total valence electrons concept is applied to draw the lewis structure of SO32- and resonance structures are drawn as well. The following steps are followed in the tutorial to draw the lewis structure of SO32-. 

AXN formula 

Using the AXN method, the molecular shape or geometry of a molecule can be easily estimated. Hence, the AXN formula is used to calculate the valence electrons and bond pairs present in the central sulfur atom of the sulfite [SO3]2- and the number of lone pairs in each. 

In the Lewis structure of SO32-, the total valence electrons present in the concerned elemental atoms is 13 and the number of bond pairs is 4. The lone pair of electrons on the central sulfur atom distorts the electronic geometry and shape of the ion making it adopt a different shape from its ideal electron pair geometry. This is a result of lone pair-bond pair electronic entropy which creates a trigonal pyramidal shape and asymmetrically enhances the polarity effect in the molecule or ion.