# Calculating Molecular Geometry Angles

1. Nov 11, 2004

### relativitydude

Howdy,

I'm trying to calculate the angles between the repulsion of electrons. Well, it looks like a vector problem. Basically, can I just think of 360º between each of the individual the XY, XZ, and YZ planes. If I set everything up with generic variables to everything, will they drop out at the end so I can get the separation in degrees in the respective XY, XZ, and YZ planes, then finally taking the arc-cosine of the dot product divided by the norm of the vectors for an overall angle?

2. Nov 11, 2004

### chem_tr

Are you studying VSEPR theorem, namely Valence Shell Electron Pair Repulsion?

You may use these basic knowledge:
1. If the central atom has no non-bonding electron pair, the geometry should be an ideal one.
2. If one non-bonding electron pair is present, bonding electrons escape from it, thereby causing a deviation of geometry.
3. If more than one non-bonding electron pair is present, repulsions among non-bonding electron pairs must be neutralized first (refer to seesaw geometry of XeF2).

3. Nov 11, 2004

### relativitydude

Well, when we have two pairs thats 180º and when we have three pairs, thats 120º, thats simple. However, four pairs is ~108º

Having that extra dimension really complicates thing. I would like to know from a physics point of view via vectors on how to calculate it.

4. Nov 11, 2004

### chem_tr

Well, I'm not sure your reasoning is absolutely correct. In octet rule, we assume that a maximum of 8 electrons for atoms except hydrogen are available, so if there are three n-electrons on an atom, only one bonding will be possible. If there is four, it is essentially a noble gas like argon, etc. (not Helium; the total number is 2 in this case).

If you wonder it from physics point of view, make sure some physicists read this post by posting a small message and including the url to this thread.