Transcript VSPER Theory

Molecular Geometry
VSEPR Theory
Who cares about Molecular
Shape?
The shape of a molecule plays a very
important role in determining its
properties.
 Properties such as smell, taste, and
proper targeting (of drugs) are all the
result of molecular shape.
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What Is The VSEPR Theory?
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VSEPR Theory -predict the shapes of molecules.
bonded pairs (shared)
lone pairs (non-bonded, unshared)
BP & LP are negatively charged clouds that repel
each other.
Stability results in clouds being as far apart as
possible in 3-D
The amount of repulsion can be ordered:
LP-LP > LP-BP > BP-BP
In order to determine the shape, the Lewis
diagram must be drawn first.
2 Bond Pairs/Electron Groups
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2 bonding pairs on the central atom =LINEAR
SHAPE
bond angle = 180°
e.g. BeF2, CO2, CS2
General Formula: AX2
Central atom A from group 2; 2 BP 0 LP
F
Be
F
Example BeF2
180°
3 Bond Pairs/Electron Groups
3 bonding pairs on the central atom =
TRIGONAL PLANAR SHAPE
 bond angles = 120°.
 e.g. BF3, BH3
 General Formula: AX3
 Central atom A from group 13;
3 BP 0
LP
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Example BF3
F
F
B
F
120°
4 Bonding Pairs/Electron Groups
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Central atom at the center of a sphere, four
pairs of electrons will occupy a position as far
apart as possible.
Result = electron pairs being at the corners of a
regular tetrahedron = TETRAHEDRAL SHAPE.
The angle between each bond will be 109.5°
e.g. CCl4, CH4, SiH4
General Formula: AX4
Central atom A from group 14;
4 BP 0 LP
Example CCl4
Cl
C
Cl
Cl
Cl
Cl
C
Cl
109.5 °
Cl
Cl
3 Bonding Pairs & 1 Non-bonding Pair
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Tetrahedral arrangement around the central atom.
Shape determined by the arrangement of the
atoms
Result = TRIGONAL PYRAMIDAL shape.
Due to the repulsion, a non-bonding electron pair
requires more space than a bonding pair
Bond angles = 107°
e.g. NH3, PCl3
General Formula: AX3E
Central atom A from group 15;
3 BP 1 LP
Example NH3
N
H
H
H
2 Bonding Pairs & 2 Non-bonding Pairs
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4 pairs of electrons arranged tetrahedrally, 2 BP =
atoms at 2 corners of the tetrahedron.
Result = V-SHAPE or BENT.
Repulsion between the non-bonding pairs will result
in a bond angle of 104.5°.
For each pair of non-bonding electrons, the bond
angle decreases by 2.5°
e.g. H2O, H2S, OCl2
General Formula: AX2E2
Central atom A from group 16;
2 BP 2 LP
H
Example H2O
O
H
Things to Remember
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Predicting the shape:
draw the Lewis Dot Diagram
 determine the number of BP and NBP
compare this with the chart you have been
given (the shapes must be memorised).
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When determining the shape of a
molecule with multiple bonds, treat the
multiple bonds as if they were single
bonds (i.e. one bonding pair)