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### Angles of molecular shapes

Posted: Fri Oct 21, 2016 6:32 pm
A molecule like BF3, or Boron Triflouride, has angles of 120 degrees, which makes sense considering it's trigonal planar so the 3 F atoms are equally spaced out around the Boron.
But for a molecule like CH4, methane, the H-C-H angles are 109.5, which seems a bit random. I visually see why that makes sense but I was wondering if there is a way we calculate these angles in degrees for all shapes or are they just facts that we should know?

Thank you!

### Re: Angles of molecular shapes

Posted: Fri Oct 21, 2016 6:39 pm
If I recall correctly, these angles are experimentally determined, so I don't think we can calculate them. Just something we need to know.

### Re: Angles of molecular shapes

Posted: Fri Oct 21, 2016 9:42 pm
To clarify, is the electron density the factor behind the bending of the angles of the bonds?

### Re: Angles of molecular shapes

Posted: Fri Oct 21, 2016 10:23 pm
Yes, the presence of electrons in bonds and especially lone pairs of e- is what determines the shape (tetrahedral, trigonal planar, trigonal bipyramidal)

### Re: Angles of molecular shapes

Posted: Sun Oct 23, 2016 3:54 pm
You actually could calculate that the bond angles of a tetrahedral molecule are 109.5 degrees, however, it would require thinking in three dimensions--aka multivariable calculus--to prove it. Same pythagorean theorem as before, but working in terms of xyz to maximize the distance between endpoints (the hypotenuse, so to speak).

Another thing to keep in mind is that lone pairs of electrons occupy more space than bonded electrons. This explains why the bond angle of NH3 is less than that of CH4: the lone pair electrons on N in ammonia repel the other bonding pairs of electrons, pushing them slightly closer together.

### Re: Angles of molecular shapes

Posted: Sun Oct 23, 2016 4:29 pm
So, does the type of bond (single, double, triple) also play a factor in the repulsion of e- and the size of the bond angles in general or are they completely determined by the lone pairs?