VSEPR THEORY AND MOLECULAR GEOMETRY Mr. Chapman Chemistry 20
Feb 23, 2016
VSEPR THEORY AND MOLECULAR GEOMETRY
Mr. ChapmanChemistry 20
Back to the Basics It seems like we have known since the
beginning of our education that opposites attract and that particles of the same charge repel each other.
This simple fact is the basis for why molecules form in the shapes that they do, and it is called VSEPR theory. VSEPR stands for Valence Shell Electron Pair Repulsion Theory.
Molecular Shape Molecules have definite shapes and the shape of a
molecule controls some of its chemical and physical properties.
What is the difference between a chemical and physical property?
There are some important definitions we must understand in order to comprehend molecular geometry.
Definitions Bond lengths – the distances between the
nuclei of bonded atoms
Bond angles –is the angle formed between three atoms across at least two bonds.
Bond lengths and bond angles are determined by the pairs of electrons that are in the valence shell of the central atom.
Molecules Can Take on a Number of Different Shapes
Valence-Shell Electron-Pair Repulsion (VSEPR) Theory Electron pairs repel each other, whether they are
in chemical bonds (bond pairs) or unshared (lone pairs).
Electron pairs assume orientations about an atom to minimize repulsions.
In other words, electrons in the valence shell want to get as far away from each other as possible.
Important Point There is an important difference
between electron pair arrangement and molecular geometry, which we are going to see in the next few slides.
Electron pair arrangement determines the molecular geometry of the molecule.
Valence shell electron pair repulsion (VSEPR) model:
Predict the geometry of the molecule from the electrostatic repulsions between the electron (bonding and nonbonding) pairs.
AB2 2 0
Class
# of atomsbonded to
central atom
# lonepairs on central atom
Arrangement of electron pairs
MolecularGeometry
10.1
linear linear
B B
Cl ClBe
2 atoms bonded to central atom0 lone pairs on central atom 10.1
AB2 2 0 linear linearClass
# of atomsbonded to
central atom
# lonepairs on central atom
Arrangement of electron pairs
MolecularGeometry
VSEPR
AB3 3 0 trigonal planar
trigonal planar
10.1
10.1
AB2 2 0 linear linearClass
# of atomsbonded to
central atom
# lonepairs on central atom
Arrangement of electron pairs
MolecularGeometry
VSEPR
AB3 3 0 trigonal planar
trigonal planar
10.1
AB4 4 0 tetrahedral tetrahedral
10.1
AB2 2 0 linear linearClass
# of atomsbonded to
central atom
# lonepairs on central atom
Arrangement of electron pairs
MolecularGeometry
VSEPR
AB3 3 0 trigonal planar
trigonal planar
10.1
AB4 4 0 tetrahedral tetrahedral
AB5 5 0 trigonalbipyramidal
trigonalbipyramidal
10.1
AB2 2 0 linear linearClass
# of atomsbonded to
central atom
# lonepairs on central atom
Arrangement of electron pairs
MolecularGeometry
VSEPR
AB3 3 0 trigonal planar
trigonal planar
10.1
AB4 4 0 tetrahedral tetrahedral
AB5 5 0 trigonalbipyramidal
trigonalbipyramidal
AB6 6 0 octahedraloctahedral
10.1
Class
# of atomsbonded to
central atom
# lonepairs on central atom
Arrangement of electron pairs
MolecularGeometry
VSEPR
AB3 3 0 trigonal planar
trigonal planar
AB2E 2 1 trigonal planar bent
10.1
Class
# of atomsbonded to
central atom
# lonepairs on central atom
Arrangement of electron pairs
MolecularGeometry
VSEPR
AB3E 3 1
AB4 4 0 tetrahedral tetrahedral
tetrahedral trigonal pyramidal
10.1
Class
# of atomsbonded to
central atom
# lonepairs on central atom
Arrangement of electron pairs
MolecularGeometry
VSEPR
AB4 4 0 tetrahedral tetrahedral
10.1
AB3E 3 1 tetrahedral trigonalpyramidal
AB2E2 2 2 tetrahedral bent
H
O
H
Class
# of atomsbonded to
central atom
# lonepairs on central atom
Arrangement of electron pairs
MolecularGeometry
VSEPR
10.1
AB5 5 0 trigonalbipyramidal
trigonalbipyramidal
AB4E 4 1 trigonalbipyramidal
distorted tetrahedron
Class
# of atomsbonded to
central atom
# lonepairs on central atom
Arrangement of electron pairs
MolecularGeometry
VSEPR
10.1
AB5 5 0 trigonalbipyramidal
trigonalbipyramidal
AB4E 4 1 trigonalbipyramidal
distorted tetrahedron
AB3E2 3 2 trigonalbipyramidal T-shaped
ClF
F
F
Class
# of atomsbonded to
central atom
# lonepairs on central atom
Arrangement of electron pairs
MolecularGeometry
VSEPR
10.1
AB5 5 0 trigonalbipyramidal
trigonalbipyramidal
AB4E 4 1 trigonalbipyramidal
distorted tetrahedron
AB3E2 3 2 trigonalbipyramidal T-shaped
AB2E3 2 3 trigonalbipyramidal linear
I
I
I
Class
# of atomsbonded to
central atom
# lonepairs on central atom
Arrangement of electron pairs
MolecularGeometry
VSEPR
10.1
AB6 6 0 octahedraloctahedral
AB5E 5 1 octahedral square pyramidal
Br
F F
FF
F
Class
# of atomsbonded to
central atom
# lonepairs on central atom
Arrangement of electron pairs
MolecularGeometry
VSEPR
10.1
AB6 6 0 octahedraloctahedral
AB5E 5 1 octahedral square pyramidal
AB4E2 4 2 octahedral square planar
Xe
F F
FF
Predicting Molecular Geometry1. Draw Lewis structure for molecule.2. Count number of lone pairs on the central atom and number
of atoms bonded to the central atom.3. Use VSEPR to predict the geometry of the molecule.
What are the molecular geometries of SO2 and SF4?
SO O
AB2E
bentS
F
F
F F
AB4E
distortedtetrahedron
10.1
Examples / Practice: Draw Lewis structures, build models, and predict
the shape of the following molecules:
1. Carbon tetrahydride2. Boron Trichloride3. Carbon dioxide (carbons and oxygens double
bonded to each other)4. Selenium dibromide5. Water6. Nitrogen trifluoride