MOLECULAR GEOMETRY
How can molecular geometry (shape) be predicted ?
VSEPR THEORY
Valence Shell Electron Pair Repulsion Theory
The electron pairs (both bonding and non-bonding) surrounding a central
atom are positioned as far as possible form each other, thus minimizing
electron-pair repulsions.
Number of Pairs
Arrangement of Pairs
Geometry
2
Linear
3
Trigonal Planar
4
Tetrahedral
6
Number of Pairs
Arrangement of Pairs
Geometry
5
Trigonal bipyramidal
6
Octahedral
7
Lewis formula
Electron Pairs
Total Bonding Lone
Electron Pair
Geometry
Molecular
Geometry
Example
AX2
X:A:X
2
2
0
FBeF
Linear
Linear
Bond Angle = 1800
O=C=O
Nonpolar
Treat double
bonds as single
bonds
AX3
F
X
..
A:X
..
X
B
3
3
0
Trigonal
planar
8
Bond Angle = 1200
Nonpolar
Trigonal Planar
F
F
Lewis formula
AX2E
E = lone pair
..
X : A :: X
or
..
X :: A : X
Treat double
bonds as single
bonds
AX4
X
..
X:A:X
..
X
Electron Pairs
Total Bonding Lone
Electron Pair
Geometry
Molecular
Geometry
Example
S
3
2
1
O
Trigonal
planar
O
Bent (angular)
Bond Angle  1200
Polar
Cl
C
4
4
0
Cl
Tetrahedral
9
Tetrahedral
Bond Angle  109.50
Nonpolar
Cl Cl
Lewis formula
AX3E
E = lone pair
..
X:A:X
..
X
Electron Pairs
Total Bonding Lone
4
3
Electron Pair
Geometry
Example
1
N
H
Tetrahedral
AX2E2
..
X:A:
..
X
Molecular
Geometry
H H
Trigonal Pyramidal
Bond Angle 109.50
Polar
O
4
2
2
H
Tetrahedral
10
Bent (Angular)
Bond Angle 109.50
Polar
H
Lewis formula
Electron Pairs
Total Bonding Lone
Electron Pair
Geometry
Molecular
Geometry
Example
AX5
Cl
X
X
Cl
X
A
Cl
5
5
P
0
Cl
Cl
X
X
Trigonal
bipyramidal
Trigonal
bipyramidal
Bond Angles:
1200 (3) and 900 (6)
Nonpolar
PCl5
SF4
AX4E
F
F
X
..
X:A:X
..
X
S
F
5
4
1
Trigonal
bipyramidal
11
Seesaw
Bond Angles:
1200, 900, and 1800
Polar
F
Lewis formula
Electron Pairs
Total Bonding Lone
Electron Pair
Geometry
Molecular
Geometry
Example
AX3E2
X
..
X:A
..
X
ClF3
F
F
5
3
2
Trigonal
bipyramidal
T-shaped
Bond Angles: 900
Polar
AX2E3
X
..
A
..
X
Cl
F
XeF2
F
5
2
3
Xe
Trigonal
bipyramidal
12
Linear
Bond Angle = 1800
Nonpolar
F
Lewis formula
Electron Pairs
Total Bonding Lone
Electron Pair
Geometry
Molecular
Geometry
Example
AX6
SF6
X
X
F
X
A
X
F
6
6
F
0
S
X
F
X
Octahedral
F
Octahedral
Bond Angle = 900
Nonpolar
AX5E
F
IF5
F
X
X
F
X
6
5
1
I
A
X
F
X
F
Octahedral
13
Square pyramidal
Bond Angle: 900
Polar
F
Lewis formula
Electron Pairs
Total Bonding Lone
Electron Pair
Geometry
Molecular
Geometry
Example
AX4E2
X
X
A
X
XeF4
F
6
X
4
2
F
Xe
Octahedral
14
Square planar
Bond Angle: 900
Nonpolar
F
F
SUMMARY
Symmetrical Arrangement
Asymmetrical Arrangement
Polar Bonds
Nonpolar Molecule
Polar Bonds Polar Molecule
Linear
Bent
Trigonal Planar
planar
Trigonal
pyramidal
Tetrahedral
Seesaw
Trigonal
bipyramidal
T-shaped
Octahedral
Square
pyramidal
Square
planar
15
BOND ANGLES
Can be approximately predicted from the VSEPR model.
Some deviations from the predicted bond angles have been determined
experimentally. These deviations are caused by 2 factors:
1. Effect of Lone Pairs
A lone pair tends to require more space than a bonding pair
Reason:
A lone pair of electrons is attracted to only one atomic core,
whereas a bonding pair is attracted to two.
The lone pair is larger, while the bonding pair is drawn
more tightly to the nuclei.
Lone pair
Bonding Pair
Lone pairs repel each other stronger than Bonding Pairs.
Result:
The repulsions between electron pairs depend on the type of electron
pairs involved.
REPULSION INCREASES
Bonding
Pairs
Bonding
Pairs
Bonding
Pairs
Weakest Repulsion
Lone
Pairs
Lone
Pairs
Lone
Pairs
Strongest Repulsion
16
CH4
Electron
Pair
Geometry
Expected
Bond Angle
Tetrahedral
NH3
Tetrahedral
H2O
Tetrahedral
109.50
109.50
109.50
Bonding
Pairs
4
3
2
Lone Pairs
0
1
2
Actual
Bond Angle
109.50
1070
17
1050
2. Effect of Multiple Bonds
Muiltiple bonds require more space than single bonds because of the
greater number of electrons:
Electron Pair Geometry:
Trigonal Planar
Expected Bond Angle:
1200
Actual Bond Angle:
1160
Reason: C = O
CH
Stronger Repulsion
CH
CH
Weaker Repulsion
18
Sample Problems
I. Predict the geometry of the SCl2 molecule.
Is the molecule polar ?
Step 1 :Write the electron-dot formula.
1 S = 1 x 6 electrons = 6 electrons
2 Cl = 2 x 7 electrons = 14 electrons
Total:
20 electrons
.. .. ..
: Cl : S : Cl :
.. .. ..
Step 2:
Step 3:
Determine how many electrons are there around the central
atom ?
4
How many bonding pairs? 2
How many lone pairs?
2
Type of molecule ?
AX2E2
Step 3:
Detemine the electron pair geometry for 4 electron pairs
Tetrahedral
Step 4:
Sketch the molecule surrounded by both bonding and
nonbonding electron pairs
19
Step 5:
Obtain the molecular geometry from the directions of the
bonding pairs.
Sketch the molecule again, but ignore the lone pairs.
Answer: The molecule has a bent (angular) gemetry and it is POLAR.
II. Predict the geometry of the COCl2 molecule.
1 C = 1 x 4 electrons = 4 electrons
1 O = 1 x 6 electrons = 6 electrons
2 Cl = 2 x 7 electrons= 14 electrons
Total:
24 electrons
:O:
..
..
O
..
..
: Cl : C : Cl :
..
..
..
: Cl
..
C
..
Cl :
..
Note: Count a double bond as one pair (one group of electrons)
Number of electron pairs?
Number of bonding pairs?
Number of lone pairs ?
3
3
0
20
Note: When there are no lone pairs, the Electron Pair Geometry and
the Molecular Geometry are the same.
The Geometry for 3 electron pairs is Trigonal Planar
O
C
Cl
21
Cl