Molecular%20Geometry%20VSEPR%20Theory - PowerPoint PPT Presentation
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Molecular%20Geometry%20VSEPR%20Theory
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Molecular Geometry VSEPR Theory Section 12.3 Trigonal Planar 3 things Bond Angle = 120 Ex: BCl3 Tetrahedral 4 things Bond Angle = 109.5 Ex: CH4 Trigonal ... – PowerPoint PPT presentation
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Title: Molecular%20Geometry%20VSEPR%20Theory
1
Molecular Geometry VSEPR Theory
- Section 12.3
2
VSEPR
- Valence Shell Electron Pair Repulsion
- Electron pairs around central atom arrange
themselves to be far apart as geometrically
possible. - Good help sites http//www.chem.purdue.edu/gchelp
/vsepr/ - http//www.classzone.com/cz/books/woc_07/resources
/htmls/ani_chem/chem_flash/popup.html?layeractsr
cqtiwf_act065.1.xml
3
Linear
- 2 things
- Bond Angle 180o
- Ex CO2
180o
4
Trigonal Planar
- 3 things
- Bond Angle 120 ?
- Ex BCl3
120o
5
Tetrahedral
- 4 things
- Bond Angle 109.5 ?
- Ex CH4
109.5o
6
Unshared e-
- Unshared pairs of electrons are extremely close
to the central atom compared to atoms forming
bonds - All the things are in the same orientation
around central atom, but the only things we see
in the molecule are the atoms forming bonds
7
Unshared e- (continued)
- For example NH3
- Draw the electron dot structure
- Determine how many things and also the shape
based on things?
8
Unshared e- (continued)
- Therefore with NH3 we expect to see this because
it has four things - But one of these spheres would be replaced by a
pair of e- REALLY close to the central atom, so
we actually see this
9
Trigonal bipyramidal
- 5 things trigonal bipyramidal
- Bond angles (90?, 120?, 180?)
- Ex. SF5
10
Octahedral
- 6 things
- Bond angles (90 ?, 180?)
- Ex.PF6
11
Steps
- Draw electron dot or Lewis structure for
molecule/ion. - Count things around central atom.
- bonds (single, double, or triple each just count
as one) - non-bonding pairs (aka unshared pairs, lone
pairs) - These things are more technically called
regions of electron density
12
Steps cont.
- Name the arrangement (5 possibilities)
- Things Arrangement
- 2 linear
- 3 trigonal planar
- 4 tetrahedral
- 5 trigonal bipyramidal
- 6 octahedral
13
Steps cont.
- a. If the central atom has no unshared pairs,
this is the name of the geometry. - Ex CH4 has 4 things so its tetrahedral
- b. If the central atom has unshared pairs,
arrange them as far apart as possible. - Doesnt matter for 2, 3, or 4 things because of
symmetry - When 5 things, lone pairs go on equatorial
positions first (since they are farther away from
other stuff) - When 6 things, lone pairs go across from each
other. - Name the molecular geometry based on the bonds.
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