Transcript Molecular Geometry VSEPR Theory

Molecular Geometry
VSEPR Theory
Section 12.3
VSEPR
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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/pop
up.html?layer=act&src=qtiwf_act065.1.xml
Linear
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2 things
Bond Angle = 180o
Ex: CO2
180o
Trigonal Planar
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3 things
Bond Angle = 120 
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Ex: BCl3
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Tetrahedral
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4 things
Bond Angle = 109.5 
Ex: CH4
H
C
H
H
H
Unshared e
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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
Unshared e- (continued)
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For example: NH3
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Draw the electron dot structure
Determine how many “things” and also
the shape based on “things”?
N
H
H
H
Unshared e- (continued)
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Therefore with NH3 we expect to see this
because it has four things:
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But one of these spheres would be replaced
by a pair of e- REALLY close to the central
atom, so we actually see this:
Trigonal bipyramidal
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5 things = trigonal bipyramidal
Bond angles: (90, 120, 180)
Ex.: SF5
Octahedral
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6 things
Bond angles: (90 , 180)
Ex.:PF6
Steps:
1.
2.
Draw electron dot or Lewis structure
for molecule/ion.
Count “things” around central atom.
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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”
Steps cont.:
3.
Name the arrangement (5 possibilities)
 Things
Arrangement
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2
linear
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3
trigonal planar
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4
tetrahedral
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5
trigonal bipyramidal
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6
octahedral
Steps cont.:
4.
a. If the central atom has no unshared
pairs, this is the name of the geometry.
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Ex: CH4 has 4 “things” so its tetrahedral
b. If the central atom has unshared pairs,
arrange them as far apart as possible.
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5.
Doesn’t 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.