(Polar molecules, Non-polar molecules, etc.)
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This is because the sulfur atom in the sulfite ion has a lone pair, and the lone pair present causes electron-electron repulsion, and therefore forces all the other bonds to be just slightly closer to each other. This is why the bond angle is 106, slightly less than the standard value of 109.5, which is the bond angle one would find in a tetrahedral with no lone pairs.
Remember that in a sulfite ion there is a lone pair of electrons attached to the sulfur atom. Because lone pairs are more repulsive than atoms, they slightly change the bond angles between the atoms to slightly less than 109.5 degrees. We don't have know the exact bond angles, we just say that they are slightly less or slightly more than whatever the bond angle in question is. At least I think so.
Since lone pair electrons have stronger repulsion force, bonded atoms will be pushed further from it and gather closer to each other. This is more stable, since atom-atom repulsion is weaker than lone pair-atom repulsion. We don't need to memorize that the particular bond angle is 106 degrees. We only need to be able to understand that this structure has a lone pair, and thus the bond angles must be smaller than the expected tetrahedral bond angle (109.5)
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