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The tetrahedral bond angles become distorted when the central atom has lone pair(s) of electrons. Since the repulsion strength between a lone [air and bonding pair of electrons is greater than bonding-bonding pairs, the lone pair of electrons force the bonding electrons closer together. Thus the bond angles become smaller.
For example, in the case of a trigonal pyramidal shape, in which there are 3 bonding pairs and one lone pair, the lone pair- bonding pair repulsion is stronger than a bonding pair- bonding pair repulsion. This condenses the angle to less than 109.5. The tetrahedral shape lacks this lone pair-bonding pair repulsion, allowing the angle to be 109.5.
We can tell when a VSEPR model has distortion because usually the central atom will have a lone pair or more. When the central atom has a lone pair, it pushes the bond electrons closer to one another, thereby making the bond angle smaller.
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