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# AS - Bond Angles Watch

1. Hi, I've got a couple of questions about bond angles. I'm studying unit 1 of AQA Chemistry, and I'm a bit confused:

How do you know how squeezed the regular bond angles are by a lone pair? It says at one point "the bonding pairs are pushed together and the bond angle is 107.5", but I wouldn't be able to work that out myself. Then when there are two lone pairs, the angle is 104.5 degrees. Why? I know it's because of lone pair-lone pair repulsion being stronger, but why that specific amount? Do I have to be able to work that out?

tl;dr: How much does a lone pair squeeze the bond angle by?

Thanks
2. Just learn the bond angles. You know that lp-lp is stronger than lp-bp so you are fine
3. (Original post by gunit123)
Just learn the bond angles. You know that lp-lp is stronger than lp-bp so you are fine
Okay, thanks. It looks like it squeezes by 2.5 degrees, but I'll just learn the angles.

Also, do I need to learn all the shapes - Beryllium Chloride, Boron trichloride, Methane,. Ethane, Ethene, Ammonia, Water, Phosphorus (V) Fluoride, Sulfur Hexafluoride - that are listed in the book?
4. (Original post by tcrwt)
tl;dr: How much does a lone pair squeeze the bond angle by?

Thanks
It'll vary considerably depending upon the bonds and atoms in question, so there isn't really any hard and fast rule that allows you to mathematically work them out (short of probably some very complicated quantum mechanics). For example NH3 is 107.5, but NF3 is 102.3.

For A-level there aren't that many "bent" molecules you have to remember (H2O, NH3, cant remember any others off the top of my head), so best bet is to memorise the bond angles for those rather than make it more complicated!
It'll vary considerably depending upon the bonds and atoms in question, so there isn't really any hard and fast rule that allows you to mathematically work them out (short of probably some very complicated quantum mechanics). For example NH3 is 107.5, but NF3 is 102.3.

For A-level there aren't that many "bent" molecules you have to remember (H2O, NH3, cant remember any others off the top of my head), so best bet is to memorise the bond angles for those rather than make it more complicated!

Thank you!
6. In my textbook it says one lone pair reduces the bond angle by approx. 2 degrees. So two lone pairs reduce it by around 4 degrees.

I did a past paper the other day with one of the questions related to this, and they let you have anything within a certain boundary (e.g. with water, they would let you get away with the bond angle being between 104-105.5 ish).

And when it comes to memorising the shapes of molecules, I wouldn't bother (well, excluding Ammonia and water which come up a lot), just learn how to work them out by memorising all the shapes and angles - linear, trigonal planar, tetrahedral, trigonal bipyramidal, octahedral.
7. (Original post by Accalia)
In my textbook it says one lone pair reduces the bond angle by approx. 2 degrees. So two lone pairs reduce it by around 4 degrees.

I did a past paper the other day with one of the questions related to this, and they let you have anything within a certain boundary (e.g. with water, they would let you get away with the bond angle being between 104-105.5 ish).
That's awesome, thanks! I was hoping there would be a bit of flexibility with how much accuracy they require

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