Is SO3 Polar or Nonpolar?

Answer: SO3(2-) is a polar molecule due to the lone pair electrons at the "top" of the structure causing electron-electron repulsion and a region of partial negative charge. The resulting bent structure leads to an unequal distribution of charge rotating between the different oxygen molecules depending on which is double-bonded to sulfur and which contain single-bonds at any given moment.  

In this article we will be discussing Sulfites that contain a 2- charge as opposed to Sulfur Trioxide which relies on an expanded octet in order to complete its configuration. The latter has no charge either formal or dipole moment since it is a nonpolar molecule with a trigonal planar configuration (i.e. 120˚ between the outer molecules). The former is described by the description right above. SO3(2-) has three resonance structures with one being shown in the diagram below. The other structures move the double bond to another part of the atom with all three positions sharing this extra bond for 1/3 of the time (so each oxygen atoms theoretically has 4/3 of a bond to the central sulfur atom). Although theoretically an acid exists by combining hydrogen atoms, this has only ever been observed in the gas phase of the molecule.

SO3 Ball and Stick Diagram
SO3 Ball and Stick Diagram. Created with Avagadro.
How is SO3(2-) utilized in the real world?

Sulfites are oftentimes utilized as food additives. They occur naturally in wines, although additional sulfites may be added in order to arrest fermentation and preserve the wine for a long period of time. They may also be utilized as a preservative for other long-lasting consumables such as dried fruit. SO3(2-) oftentimes does not appear in its pure form; rather, it is added ionically-bonded to a salt such as sodium, potassium or calcium. It has been noted that there are specific allergic effects and a higher risk for those with asthma or other respiratory conditions. It is generally regulated that sulfite content be labelled on the packaging of food items. 

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