|MadSci Network: Chemistry|
I'm assuming that you have some knowledge of Lewis Structures as a background. VSEPR theory attempts to explain the 3-D structure of molecules based on the arrangement of electron pairs around a central atom. This goes back to the idea that like charges repel. In this case, pairs of electrons (which are negatively charged) repel other electron pairs with like charges. The molecular configurations which are stable manage to place electron pairs as far apart as possible.
That's the basic idea; now let's get down to the specifics. You begin by drawing a Lewis Structure of the molecule in question. For simplicity's sake, let's use water, H2O, as the example. Draw a Lewis Structure for water, being careful to place the lone pairs around the central oxygen atom.
.. :O - H | HNow, count up the number of electron pairs -- regardless of whether they're in a bond or if they're lone pair electrons -- around the central atom. You should have a structure with 2 bonding pairs (in single O-H bonds) and 2 lone pairs. Since your molecule has a total of 4 electron pairs around the oxygen, the electron pairs repel each other until they assume a TETRAHEDRAL electron geometry. A tetrahedron with bond angles of about 109 degrees maximizes the distance between adjacent electron pairs. You can also find the molecular geometry of this molecule, or the arrangement of the ATOMS (not just the electron pairs) in 3 dimensions. To do that, you compare the number of lone pair electrons and the number of bonding electrons. 2 lone pairs and 2 bonding pairs gives you a bent shape, like a pair of Mickey Mouse ears. A water molecule assumes a bent shape because the lone pair electrons repel each other quite strongly, and this distorts the tetrahedron a little. It's easy to confuse electron and molecular geometry --- with electron geometry you're visualizing all of the electron pairs, but with molecular geometry you're only paying attention to bonding pairs and the attached atoms.
You may encounter double or triple bonded compounds. In that case, count ANY bond (single, double, triple) as just one electron pair. Also, you may be looking at compounds with more than one central atom. When considering VSEPR structures, only focus on one central atom at a time. So, for a molecule like CH3CH2OH (ethanol), analyze each carbon and the oxygen separately.
Your book probably contains a chart of electron geometries and molecular geometries, but just in case it doesn't, here's a short summary. Sorry about the lack of illustrations!
# electron # bonding # lone electron geo. molecular geo. example pairs pairs pairs ************************************************************************ 2 1 1 linear linear CO 2 0 linear linear CO2 3 3 0 trigonal planar trig. planar BH3 2 1 trig. planar bent O3 4 4 0 tetrahedral tetrahedral CH4 3 1 tetrahedral trig. pyramid NH3 2 2 tetrahedral bent H2O 5 5 0 trig. bipyramid trig. bipyramid PF5 4 1 trig. bi "seesaw" SF4 3 2 trig. bi. "T-shape" ClF3 2 3 trig. bi linear I3- 6 6 0 octahedral octahedral SF6 5 1 octahedral square pyramid XeF4 4 2 octahedral square planar XeF2I hope this helps! Email me if you have any further questions.
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