(Polar molecules, Non-polar molecules, etc.)
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How do you determine if a bond is slightly less than 109.5 or the general bond angle? For instance, in the textbook, on problem 2E.21, for the compound N2H4, the shape is trigonal pyramidal so I assumed the bond angle would be 109.5 but the textbook is saying its 107. How would we determine this?
If there is a lone pair on the central atom, you know that the bond angle will be less than 109.5. This is because the lone pair repels the bonding electrons and pushes the rest of the atoms bonded to the central atom down. You don't have to know that it's 107 degrees. You just have to know that it is slightly less than 109.5.
As others have said, lone pairs are spread out more since they are not held between two positively charged nuclei. So if an atom has a lone pair the bond angles will be slightly less than the normal angles for that shape. You don't need to know the exact angle, just that it will be slightly less due to the lone pair.
If there are 4 regions of electron density with 3 of them being bonded pairs and one being lone pair, then the lone pair will create more repulsion, causing the bonded pairs to be pushed down, making their bonds closer together (so less than 109.5)
If the central atom has lone pairs it will cause the angle to be less than 109.5. You won't be able to figure out the exact bond angle because that is experimentally determined, but you will know that the angle is going to be less than 109.5
It depends if lone pairs are present. Because lone-lone pair repulsion strength is strongest compared to lone-bonding and bonding-bonding pairs, we know they require more space, thereby making the bond angle slightly smaller than 109.5. However, we aren't expected to calculate the exact value of the bond length; saying that it's <109.5 should suffice!
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