Which is predicted to have the shorter sulfur–oxygen bonds, #SO_3# or #SO_3^(2-)# ?

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Answer 1 · 2015-08-25T01:52:08

Sulfite ion has the shorter #"S-O"# bonds.

The Lewis structure of #"SO"_3# is

It has a total of six σ and π bonds to the three #"O"# atoms.

The average bond order of an #"S-O"# bond is #6/3=2#.

The Lewis structure of #"SO"_3^(2-)# is

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It is a resonance hybrid, with a total of four σ and π bonds to the three #"O"# atoms.

The average bond order of an #"S-O"# bond is #4/3≈1.33#.

Since bond length decreases as bond order increases, #"SO"_3# is predicted to have the shorter bond length.