Molecular geometry and electronic geometry help

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Nov 29, 2009
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I dont get it. The Blue book has a page on it. When the heck do you know what to use and so on? I need a explain it to dummies version. What do we use first? Ack.

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I did Chad's videos. I'm still not sure that's why I asked.

So from what I understand.

basically, the SP (sp1, sp2, sp3) are linear, trig planar, tetrahedral. Right?

The electronic geometry are the weird ones like pyramidal???? Because they count the lone electron pairs and Geometry doesnt. Am I right?

That damn calgary website basically says that the electronic and geometrical geometries are the same, from what I can tell.
basically look at how many groups are surrounding an atom
a group can either be a lone pair of electrons or another atom bonded to the atom you're looking at
4 groups surrounding and it's sp3, 3 groups sp2, 2 groups sp
for the molecular shape you have to look at the identity of these groups and such
for example CO2 (sp) which has carbon in the middle double bonded to oxygen on either side is linear, CH4 is tetrahedral, NH3 (note that the N is still sp3 hybridized but not tetrahedral) is trigonal pyramidal while BF3 is trigonal planar (sp2 because the B has no lone pair)
^oh and also dont forget that electron geometry is something different from molecular geometry for example i believe NH3^^ cited above actually has a tetrahedral electron geometry but a trigonal pyramidal molecular geometry

Thanks. So basically dont forget to count the electron pairs if a molecule has it and then determine the shape based on that? Is that all? I thought geometry means to count just the atoms and then electronic meant to count the atoms + the lone pairs. :rolleyes:
Think of them as 3 separate things. To get the hybridization (sp, sp2, sp3) do like i stated above^ which is to count the groups surrounding the atom you're looking at, very simple. However the same hybridization doesn't always mean the same molecular and electron geometry. As the example of NH3 and CH4 shows (both N and C in those compounds are sp3 hybridized, however NH3's molecular geometry is trigonal pyramidal and CH4's molecular geometry is tetrahedral) Molecular and electron geometry are not so simple and I don't remember everything off the top of my head lol. Might be a good idea to pick up a book and review those but as for hybridization if you use what i said you'll be good =]
Ok so I actually just reviewed everything on VSEPR so I'm gonna try to explain it better now:
1. Count number of groups surrounding central atom (or atom of interest) in a molecule. A group=lone pair or another atom bonded to the atom of interest (Keep in mind that it doesn't matter if it's single or triple bonded it still only counts as ONE group)
2. If you have 4 groups surrounding, sp3 hybrid orbitals and tetrahedral electron geometry. If you have 3 groups surrounding, sp2 hybrid orbitals and trigonal planar electron geometry. Finally, if you have 2 groups surrounding, sp hybrid orbitals and linear electron geometry. Can you have more? Of course but I doubt DAT is gonna ask us about sp3d and sp3d2 hybrid orbitals so I'll skip that.
3. Electron geometry does NOT determine molecular geometry. After determining the electron geometry now you must take into account the IDENTITY of those groups surrounding your central atom (whether they are bonding electrons or lone pairs). If you have sp hybrid orbitals and linear electron geometry, molecular geometry also is linear whether you have two atoms bonded to your central atom (CO2) or you have an atom and a lone pair (N2, which has N's triple bonded and each N also has a lone pair 180 degrees from the triple bond). If you have sp2 hybrid orbitals and trigonal planar electron geometry, you can have trigonal planar molecular geometry (if all groups are bonded atoms, ie: BF3) or bent molecular geometry (ie: SnCl2, where there is a lone pair on Sn and thus Sn has 3 groups: 2 bonding groups and one lone pair). Finally, if you have sp3 hybrid orbitals and tetrahedral electron geometry you can have tetrahedral molecular geometry (CH4 with all 4 groups as bonded atoms), trigonal pyramidal molecular geometry (NH3 with 3 groups as bonded atoms and one group as a lone pair), or bent molecular geometry (H20 with 2 groups as bonded atoms and two lone pairs). Phew!
Hope that clears everything up.