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

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Number of Pairs Arrangement of Pairs Geometry

5 Trigonal bipyramidal

6 Octahedral

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Lewis formula Electron Pairs Electron Pair Molecular Example
Total Bonding Lone Geometry Geometry

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 B
..
A:X 3 3 0 Trigonal Bond Angle = 1200 F F
.. planar Nonpolar
X Trigonal Planar

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Lewis formula Electron Pairs Electron Pair Molecular Example
Total Bonding Lone Geometry Geometry

AX2E
E = lone pair S
.. 3 2 1
X : A :: X O O
or Trigonal Bent (angular)
.. planar Bond Angle  1200
X :: A : X Polar
Treat double
bonds as single
bonds
AX4 Cl

X C
..
X:A:X 4 4 0 Cl Cl Cl
..
X Tetrahedral Tetrahedral
Bond Angle  109.50
Nonpolar

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Lewis formula Electron Pairs Electron Pair Molecular Example
Total Bonding Lone Geometry Geometry

AX3E
E = lone pair
..
X:A:X 4 3 1 N
..
X H H H
Tetrahedral Trigonal Pyramidal
Bond Angle 109.50
Polar

AX2E2
.. O
X:A: 4 2 2 H H
..
X
Tetrahedral Bent (Angular)
Bond Angle 109.50
Polar

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Lewis formula Electron Pairs Electron Pair Molecular Example
Total Bonding Lone Geometry Geometry

AX5 Cl
X Cl
X X Cl P
A 5 5 0 Cl
Cl
X X Trigonal Trigonal
bipyramidal bipyramidal PCl5
Bond Angles:
1200 (3) and 900 (6)
Nonpolar
SF4

AX4E F
F
X S
.. F
X:A:X 5 4 1 Trigonal Seesaw F
.. bipyramidal Bond Angles:
X 1200, 900, and 1800
Polar
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Lewis formula Electron Pairs Electron Pair Molecular Example
Total Bonding Lone Geometry Geometry
AX3E2 ClF3
F
X
.. F Cl
X:A 5 3 2
.. Trigonal T-shaped F
X bipyramidal Bond Angles: 900
Polar
AX2E3 XeF2

X F
..
A 5 2 3 Xe
.. Linear
X Trigonal Bond Angle = 1800 F
bipyramidal Nonpolar

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Lewis formula Electron Pairs Electron Pair Molecular Example
Total Bonding Lone Geometry Geometry
AX6 SF6

X F
X X F F
A 6 6 0 S
X X F F
X Octahedral Octahedral F
Bond Angle = 900
Nonpolar
AX5E IF5
F
X F F
X X 6 5 1 I
A F F
X X Octahedral Square pyramidal
Bond Angle: 900
Polar

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Lewis formula Electron Pairs Electron Pair Molecular Example
Total Bonding Lone Geometry Geometry
AX4E2 XeF4

X X F F
A 6 4 2 Xe
X X Octahedral Square planar F F
Bond Angle: 900
Nonpolar

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 SUMMARY
Symmetrical Arrangement Asymmetrical Arrangement
Polar Bonds Nonpolar Molecule Polar Bonds Polar Molecule

Linear Bent

Trigonal Planar Trigonal

planar pyramidal

Tetrahedral Seesaw

Trigonal
bipyramidal T-shaped

Octahedral Square
pyramidal

Square
planar

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 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 Bonding Bonding Lone Lone Lone

Pairs Pairs Pairs Pairs Pairs Pairs

Weakest Repulsion Strongest Repulsion

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 CH4 NH3 H2O

Electron
Pair Tetrahedral Tetrahedral Tetrahedral
Geometry

Expected
Bond Angle 109.50 109.50 109.50

Bonding
Pairs 4 3 2

Lone Pairs 0 1 2

Actual 109.50 1070 1050

Bond Angle

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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 CH CH

Stronger Repulsion Weaker Repulsion

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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: Determine how many electrons are there around the central
atom ?
4 How many bonding pairs? 2
How many lone pairs? 2
Step 3: 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

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 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? 3

Number of bonding pairs? 3
Number of lone pairs ? 0

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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

Cl Cl

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