Textbook 2D. 13 Part B

[Emma Chang 1G]

Place the following molecules or ions in order of decreasing bond length:

The SO bond in SO2, SO3, SO3^2-

The answer was SO3^2- > SO2 = SO3 (SO2 and SO3 bond length are about the same)

When I drew out the Lewis structures for all three of these, I thought all of them had only double bonds in their most stable structures, so I thought the bond lengths for all three would be about the same. Can anyone explain why SO3^2- has a longer SO bond than the other two?

[Chem_Mod]

SO3 2- has resonance, wherein the resonance structures have single and double bonds.

[Emma Chang 1G]

That makes sense, I forgot to account for resonance. Thank you so much!