Thread: Molecular Geometry - Bond Angles

It's getting pretty close to an exam and I find it quite hard to remember the exact angles of certain bonds so I'm wondering if there is any mathematical formula I could use to get the values for the bond angles?



The lone pairs repel more and decrease the bond angle.

Does anyone know of/can deduce a mathematical formula to get the angles?


Thanks guys!

Originally Posted by anthmoo

It's getting pretty close to an exam and I find it quite hard to remember the exact angles of certain bonds so I'm wondering if there is any mathematical formula I could use to get the values for the bond angles?



The lone pairs repel more and decrease the bond angle.

Does anyone know of/can deduce a mathematical formula to get the angles?


Thanks guys!

I doubt it. The bond angles in the table have been measured and thus are correct. There is a way to predict them using Quantum Mechanics, but as there is only an exact solution for the Hydrogen atom, the result is gotten by using an approximation technique. (I believe it's called the "Hartree-Fock" method, but don't take my word on that.) Basically you guess at what the wavefunctions should be and use that as a "seed value" of sorts to generate the next approximation. Then plug the result back into the algorithm and you get your next level approximation. etc. Eventually the input and output wavefunctions are essentially the same. This takes hours even on a supercomputer.

I suspect it would be easier just to memorize them.

-Dan

Originally Posted by topsquark

I doubt it. The bond angles in the table have been measured and thus are correct. There is a way to predict them using Quantum Mechanics, but as there is only an exact solution for the Hydrogen atom, the result is gotten by using an approximation technique. (I believe it's called the "Hartree-Fock" method, but don't take my word on that.) Basically you guess at what the wavefunctions should be and use that as a "seed value" of sorts to generate the next approximation. Then plug the result back into the algorithm and you get your next level approximation. etc. Eventually the input and output wavefunctions are essentially the same. This takes hours even on a supercomputer.

I suspect it would be easier just to memorize them.

-Dan

I only have a TI-84 Graphical Calculator and 1 hour 20 minutes in the exam!

Hahaha you have a point! I'll try to memorize them.

Thanks man!

One thing that people, I realized, misunderstand is that with math you can solve everything. You cannot. Most of the things that involve science are done by experiment and measured, that is the only way. Thus, I assume there is no way here likewise.

(What I do not understand what it means bond angles? That is what I hate about science. One place they say they are electrons, one place they say they are clouds and hence no particles another place something else. Thus, how do you explain meaning of bound angles?)

Originally Posted by ThePerfectHacker

One thing that people, I realized, misunderstand is that with math you can solve everything. You cannot. Most of the things that involve science are done by experiment and measured, that is the only way. Thus, I assume there is no way here likewise.

(What I do not understand what it means bond angles? That is what I hate about science. One place they say they are electrons, one place they say they are clouds and hence no particles another place something else. Thus, how do you explain meaning of bound angles?)

Yes, electrons are all in clouds rather than nice little orbiting charges as in the Bohr model. The bond angles can be quite easily measured since the position of the nuclei are oscillating in a very small space (compared to the electron cloud.) The nuclei are more or less stationary so you can draw lines between the different nuclei. (To be really precise we should use the average position of the nuclei.)

-Dan

ThePerfectHacker: As mentioned above, it is the angel between three nuclei.

About the table for the exam. I've seen a few of those and its intention is most probably for you to learn by heart to have a "feeling" for the geometry within the molecules. Sorry, no formula or shortcuts. Just a bit of knowhow instead of
Hartree Fock, MP2, DFT, CAS and what ever acronym those quantum chemists can come up with.