Recently when we were doing the VSEPR Theory in class and discussing the bond angles in various compounds. That's when I thought of sulfur dioxide whose structure can be like this:

Sulfur Dioxide

According to the Internet, the bond angle between the two oxygen atoms = 120°. But mathematics shows a little off. I was taught that the VSEPR Theory states that

Lone Pair-Lone Pair Repulsion > Bond Pair-LonePair Repulsion > Bond Pair-Bond Pair Repulsion

According to this, the angle between the two oxygens should be less than 120º and I can prove it.

Imagine that instead of the lone pair there was a double bond of oxygen with sulfur. This would imply that the bond angle between the oxygens would be exactly 120° on the same plane because each one of them would repel each other with the same repulsion force. But now instead of a bond pair, we have a lone pair which means (according to the VSEPR Theory) that it would repel the two oxygens much more than the imaginary bond pair was repelling. Therefore the angle between the lone pair and the bond pairs with oxygen would be more than 120°. This would result in a smaller than 120º angle between the two oxygen atoms. Then how does the net say that the bond angle is close to 120°? If what the web says is right, it implies that the repulsion of a lone pair and 2 bonds is exactly equal to the repulsion between 2 double bonds. Is it correct?


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    $\begingroup$ VSEPR is not much of a theory; it is just an empirical model which happened to work surprisingly well in many cases. Then again, according to Wikipedia, the angle is indeed slightly less than $120^\circ$, so I see no contradiction. $\endgroup$ – Ivan Neretin Feb 1 '17 at 15:37
  • $\begingroup$ Please clarify your concepts on why lone pair-lone pair repulsions are stronger than bond pair-bond pair repulsions. I think you will get your answer $\endgroup$ – YUSUF HASAN Oct 22 '18 at 10:24

From nowhere you can cited that the repulsion of a lone pair and a double bond are exactly the same. Instead, the lone pair is little closer to sulfur than the other two, as oxygen is far more electronegative compared to sulfur. As a result, the repulsion of the lone pair is little stronger than the other two; and the molecular angle is a little smaller than 120 degrees.


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