In the correct Lewis structure for SO3 shown below, there are two S-O single bonds and one S-O double bond. Explain why the S-O bond distances are all the same.

Answer 1

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The current method for drawing Lewis dot structures, in order to show the eight electrons around S and O atoms givies the illusion of single bonds and double bonds. However, in actual practice there are no signle and double bonds. The bonds are somewhere between single bonds and double bonds but all are of the same strength. That's because of resonance. The bottom line is that we don't understand how to draw the exact structure so we do the best with drawing several structures in which the double bonds and single bonds "migrate", so to speak, in which we draw the S=O and S-O bonds in one place for one image and we swap so those bonds are moved around the molecule. Let me emphasize that the molecule does NOT have three structures. It ALWAYS has only one strucure. Those three S-O bonds are the same strength. We just don't know how to draw the REAL structure and obey all of the rules we've set about filling the outer orbits with electrons etc. Summarize all of this by saying that all the S-O bonds are the same strength and it's because of resonance. We CAN draw the different resonance structures, and often do. We just don't draw the "single" resonance structure. I know this is long but I assume you may be new to resonance, as was I at one time. I misunderstood at the beginning and actually thought for a year or more that the S=O bond was that part of the time and an S-O bond part of the time. That isn't true and I don't want you to spend more than a year understanding resonance. Good luck in chemistry.