Question 8.10: Which is predicted to have the shorter sulfur–oxygen bonds, ...
Which is predicted to have the shorter sulfur–oxygen bonds, SO_{3} or SO^{2-}_{3} ?
The sulfur atom has six valence electrons, as does oxygen. Thus, SO_{3} contains 24 valence electrons. In writing the Lewis structure, we see that three equivalent resonance structures can be drawn: Our analysis of the Lewis structures thus far leads us to conclude that SO_{3} should have the shorter S—O bonds and SO^{2-}_{3} the longer ones. This conclusion is correct: The experimentally measured S—O bond lengths are 142 pm in SO_{3} and 151 pm in SO^{2-}_{3}.
As with NO^{-}_{3} the actual structure of SO_{3} is an equal blend of all three. Thus, each S—O bond length should be about one-third of the way between the length of a single bond and the length of a double bond. That is, S—O should be shorter than single bonds but not as short as double bonds. The SO^{2-}_{3} ion has 26 electrons, which leads to a dominant Lewis structure in which all the S—O bonds are single:
\left[:\overset{..}{\underset{..}O}—\overset{..}{\underset{\large\underset{:\underset{..}O:}|}S}—\overset{..}{\underset{..}O}:\right]^{2-}