(Polar molecules, Non-polar molecules, etc.)
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How could we explain why a bond angle is less than another given certain bond angle? For example, sulfite has 4 electron pairs arranged tetrahedrally and the bond angles are around 106 degrees - less than 109.5. What is the specific explanation for why this is so?
Often we can answer the less than values by determining the differences in repulsion compared to other examples of the same orientation, where greater repulsive force from a pair or atom will push the others accordingly. It depends on each unique compound.
For molecules with bond angles less than 109.5, such as sulfite, we say that the lone pair exerts a stronger force on the other bonding angles and it pushes them down. For molecules with bond angles less than 109.5 that have bonds with different elements, such as CH3Cl, we say that since the bonds are with 3 different elements, the bond angles differ slightly. In both cases, the bond angles are not 109.5.
To add on to the previous reply, for example, a molecule with a tetrahedral shape that has all bonding pairs may have bond angles of 109.5, but replace one of those bonding pairs with a lone pair and the bond angles will decrease because lone pairs repel bonding pairs more than bonding pairs repel other bonding pairs. Moreover, lone pairs repel lone pairs the most.
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