(Polar molecules, Non-polar molecules, etc.)
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Lone pairs have a more strongly repelling effect than do bonding pairs. So, when there are lone pairs the bonding pairs are pushed closer together, decreasing the bond angles between them. These bond angles are experimentally determined.
Lone pairs have very strong repulsive forces. Atoms become closer together because the repulsive forces between atoms is less than that of a lone pair and an atom. You can determine the angle by 1. Counting how many domains there are 2. View the "normal" bond angle for the shape (pretend the lone pair is an atom) 3. State that the bond angle for the structure with the lone pair is less than this value. For example, a structure with 4 total domains (one of which is a lone pair) has angles that are <109.5 and angles that are >109.5. We based the number on the bond angle value of a tetrahedral.
When you have a lone pair, it causes repulsion and it makes the bonded pairs want to get closer together which decreases the degree of the angle, also Lavelle has said he doesn't expect us to know the exact degree of the angle, as long as we know its less than the original shape, for example trigonal planar has a bond degree of 120 so if we make one of the bonded atoms a lone pair then it would become bent and we know the bonded pairs are trying to get away from the lone pair so you would put that the degree of the angle is <120. Hope this helps!
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