Question 8.15: Draw two resonance structures for sulfurous acid (H2SO3): on...
Draw two resonance structures for sulfurous acid (H2SO3): one that obeys the octet rule for the central atom, and one that minimizes the formal charges. Determine the formal charge on each atom in both structures.
Strategy Begin by drawing the skeletal structure and counting the total number of valence electrons. Use the steps outlined in Section 8.5 to draw the first structure and reposition one or more lone pairs to adjust the formal charges for the second structure.
Setup The skeletal structure is
H-O-\overset{\overset{O}{|} }{S} -O-H
Note that each hydrogen in an oxoacid is attached to an oxygen atom and not directly to the central atom. The total number of valence electrons is 26 (six from S, six from each O, and one from each H).
Following the procedure outlined on page 314, we get the first structure:
From the top O atom, which has three lone pairs, we reposition one lone pair to create a double bond between O and S to get the second structure:
Incorporating the double bond results in every atom having a formal charge of zero.