chapter 13 molecular shapes vsepr theory
TRANSCRIPT
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Chapter 13
Molecular Shapes
VSEPR Theory
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Molecular Shape
• Molecular shape or molecular geometry
is the three-dimensional arrangement of
the atoms in a molecule.
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Molecular Shape
• Molecular shape determines several
properties of a substance including:
– reactivity
– polarity
– phase of matter
– color
– magnetism
– biological activity
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Olfaction – sense of smell
• Lock-and-Key Theory: humans can
smell various odors because each three-
dimensional odor molecule fits into only
one type of receptor.
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Gustation – the sense of taste
• Taste receptors are
located on the tongue
and are sensitive to four
major tastes: salty,
sweet, sour and bitter.
• Taste receptors respond
differentially to the
varying shapes of the
food molecules that we
eat.
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H2
H H
2(1) = 2e-
Any 2 atoms
linear
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What about molecules consisting of
more than two atoms
For this we must use the VSEPR Theory
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VSEPR Theory
• Valence Shell Electron Pair Repulsion Theory
• Predicts the molecular shape of a bonded
molecule containing a central atom(s).
• Electrons around the central atom arrange
themselves as far apart from each other as
possible
• So only electrons (lone pairs or bonds)
connected to the central atom are important.
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Five Molecular Shapes
See the table in your notes
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Steps in Determining Molecular Shape
1. Draw the Lewis Structure for the
Molecule.
2. Count the number of atoms attached to
the central atom.
3. Count the number of lone pairs attached
to the central atom.
4. Use your counts on steps 2 and 3 to
determine the shape of the molecule.
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H2S
• Dihydrogen monosulfide is commonly known as
sewer gas. This colorless, toxic and flammable
gas is responsible for the foul odor of rotten eggs.
H HS••
••
2(1)+6 = 8e-
2/2
angular
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H2S
• Dihydrogen monosulfide is commonly known as
sewer gas. This colorless, toxic and flammable
gas is responsible for the foul odor of rotten eggs.
H HS
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Methane (CH4)4 + 4(1) = 8e-
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Methane (CH4)4 + 4(1) = 8e-
4 atoms attached to the
central atom.
0 lone pairs on the
central atom.
4/0
Tetrahedral shape
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Methane (CH4)4 + 4(1) = 8e-
4 atoms attached to the
central atom.
0 lone pairs on the
central atom.
4/0
Tetrahedral shape
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HCN
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HCN
H NC••
••
1+4+5 = 10e-
••
••
••
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HCN
H NC••
••
1+4+5 = 10e-
••
••
••
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HCN
H NC••
••
1+4+5 = 10e-
••
••
••
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HCN
H NC••
1+4+5 = 10e-
••
••
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HCN
H NC••
1+4+5 = 10e-
••
••
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HCN
H NC
1+4+5 = 10e-
••
2/0
linear
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H2S
H H
S
2/2
angular
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SO2
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SO2
O OS••
••
6+2(6) = 18e-
••
••
••••
••••
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SO2
O OS••
••
6+2(6) = 18e-
••
••
••••
••••
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SO2
O OS••
6+2(6) = 18e-
••
••••
••••
2/1
angular
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CH4
4/0
tetrahedral
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NO3-
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NO3-
O ON••
••
5+3(6)+1 = 24e-
••
••
••••
••••
O ••
••
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NO3-
O ON••
••
5+3(6)+1 = 24e-
••
••
••••
••••
O ••
••
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NO3-
O ON••
••
5+3(6)+1 = 24e-
••
••
••••
••••
O ••
••
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NO3-
O ON
5+3(6)+1 = 24e-
••
••
••••
••••
O ••
•• 3/0
trigonal
planer
[ ]-
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SOCl2
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Draw the Lewis structure
for an acetate ion?You learned acetate as C2H3O2
-.
However writing it as CH3CO2- will give you a
better understanding of its Lewis structure.
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Draw the Lewis structure
for an acetate ion?
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What’s the shape of acetate?
4/0
tetrahedral
3/0
trigonal
planer