Chapter 9: Molecular Geometry and Bonding Theories
What determines molecular shape?
Bond angles:
e.g. CCl4
angle formed between two adjacent
bonds on the same atom
bond angle: ?
Chapter 9: Molecular Geometry and Bonding Theories
What determines molecular shapes?
Lewis Structures
=> tell us how atoms are physically connected
No information regarding the
actual 3-D structure of molecules
Chapter 9: Molecular Geometry and Bonding Theories
Repulsion of valence electrons => largest possible separation of atoms
Valence-Shell Electron-Pair Repulsion Model
Chapter 9: Molecular Geometry and Bonding Theories
Different ways of depicting 3-D structure
bond behind of
the paper plane
"Ball and Stick"
"Spacefilling"
bonds in plane
of paper
bond in front of
the paper plane
Chapter 9: Molecular Geometry and Bonding Theories
Lewis structures show number of electron domains
nonbonding pair
Types of Electron Domains:
● nonbonding (or "lone") electrons
● single OR double OR triple bonding
electrons
=> Bonding AND non-bonding electron
pairs take up space
bonding pair
Chapter 9: Molecular Geometry and Bonding Theories
Molecular shape depends on electron domain geometry
Lewis Structures tell us …
●
●
where bonds (bonding electron pairs) are
the location of nonbonding electrons
}
around the central atom
Bonding AND non-bonding
electron pairs take up space
Chapter 9: Molecular Geometry and Bonding Theories
Possible Electron Domain Geometries:
180o
109.5o
120o
Linear
Trigonal Planar
90o
120o
Tetrahedral
90o
90o
Trigonal
Bipyramidal
Octahedral
Chapter 9: Molecular Geometry and Bonding Theories
Lewis structures → electron domain geometry → molecular geometry
4 electron domains around central atom
=> electron domain geometry: Tetrahedral
nonbonding pair
bonding pair
3 non-bonding +
1 bonding electron domain
Chapter 9: Molecular Geometry and Bonding Theories
Lewis structures → electron domain geometry → molecular geometry
4 electron domains around central atom
=> electron domain geometry: Tetrahedral
F
S
F
Chapter 9: Molecular Geometry and Bonding Theories
Lewis structures → electron domain geometry → molecular geometry
3 electron domains around central atom
=> electron domain geometry: Trigonal planar
F
F
B
F
Chapter 9: Molecular Geometry and Bonding Theories
Lewis structures → electron domain geometry → molecular geometry
2 electron domains around central atom
=> electron domain geometry: Linear
O
C
O
Chapter 9: Molecular Geometry and Bonding Theories
Lewis structures → electron domain geometry → molecular geometry
3 electron domains around central atom
=> electron domain geometry: Trigonal planar
O
N
O
Chapter 9: Molecular Geometry and Bonding Theories
Lewis structures → electron domain geometry → molecular geometry
Molecular Geometry depends on how many of the
electron domains are actually bonds
4 electron domains:
=> electron domain geometry
Tetrahedral
3 bonds, 1 nonbonding pair:
=> molecular geometry
Trigonal pyramidal
Chapter 9: Molecular Geometry and Bonding Theories
Bond angles can be distorted:
109.5
●
o
107
o
104.5
o
nonbonding electron pairs occupy more volume than bonding pairs
Chapter 9: Molecular Geometry and Bonding Theories
Bond angles can be distorted
125.3
111.4
o
125.3
●
o
o
multiple bonds occupy a larger volume than single bonds
Volume
Volume
lone pairs => triple bonds => double bonds => single bonds
Chapter 9: Molecular Geometry and Bonding Theories
Elements from the 3rd period onward..
have d -orbitals
● can have an expanded valence shell
● may have more than 4 electron domains surrounding them
●
e.g. phosphorous:
P: [Ne] 3s2 3p3
3s
3p
3d
"expanded" valence shell
Chapter 9: Molecular Geometry and Bonding Theories
Equatorial bond
Axial bond
Chapter 9: Molecular Geometry and Bonding Theories
Chapter 9: Molecular Geometry and Bonding Theories
# electron
domains
F
TeF6
F
F
Te
F
F
F
electron
domain
geometry
molecular
geometry
Chapter 9: Molecular Geometry and Bonding Theories
# electron
domains
H3O
+
+
electron
domain
geometry
molecular
geometry
Chapter 9: Molecular Geometry and Bonding Theories
# electron
domains
NO
2
O
N
O
electron
domain
geometry
molecular
geometry
Chapter 9: Molecular Geometry and Bonding Theories
# electron
domains
SO3
electron
domain
geometry
molecular
geometry
Chapter 9: Molecular Geometry and Bonding Theories
# electron
domains
SCl2
Cl
S
Cl
electron
domain
geometry
molecular
geometry
Chapter 9: Molecular Geometry and Bonding Theories
# electron
domains
I
SbI5
I
I
Sb
I
I
electron
domain
geometry
molecular
geometry
Chapter 9: Molecular Geometry and Bonding Theories
The VESPR model can be extended to larger molecules
e.g. glycine :
predicted bond angles:
?
?
?
Electron-domain geometry:
?