Chapter 10. Molecular Shapes, Valence Band, Molecular Orbital

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Chapter 10. Chemical Bonding II: Molecular Shapes, Valence Bond Theory &
Molecular Orbital Theory
I) Valence Shell Electron Pair Repulsion (VSEPR) Theory
1) VSEPR Theory provides a way of predicting molecular structure - three
dimensional arrangement of the atoms in a molecule.
2) VSEPR model is useful in predicting the molecular geometry of molecules.
3) Core Concept: Main postulate of VSEPR model is that structure around a
given atom in a molecule is determined by minimizing electron-pair repulsions.
A) How does VSEPR Work?
i) Must have correct Lewis structure drawn.
ii) Molecular Geometry is always taken from central atom’s point
of view.
iii) VSEPR designation used to assign geometry of molecule.
ABxEy
A = central atom
B = # atoms directly attached to central atom.
E = # lone pairs directly attached to central atom.
x,y = integer values
B) Match VSEPR designation with entry in VSEP table.
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Table 10.1.
# Atoms
Bonded to
Central
Atom
VSEPR Table
Molecular
Geometry
VSEPR
Designation
Examples
2
# lone
Set Total
pairs
attached to
central
atom
0
2
Linear
AB2E0
BeCl2
3
2
0
1
3
3
Trigonal planar
Angular
(V-shaped)
AB3E0
AB2E1
BF3 , NO3SnCl2
4
3
0
1
4
4
AB4E0
AB3E1
CCl4
NH3
2
2
4
Tetrahedral
Trigonal
pyramidal
Angular
(V-shaped)
AB2E2
H2 O
5
0
5
AB5E0
PCl5
4
1
5
AB4E1
TeCl4
3
2
2
3
5
5
Trigonal
bipyramidal
Irregular
tetrahedron
T-shaped
Linear
AB3E2
AB2E3
BrF3
XeF2
6
5
0
1
6
6
AB6E0
AB5E1
SF6
IF5
4
2
6
Octahedral
Square
pyramidal
Square Planar
AB4E2
XeF4
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II) Hybridization
1. Mixing of individual atomic orbitals to form “special orbitals”.
2. The number of “special orbitals” (hybridized orbitals) must always be the same
as the number of atomic orbitals.
A) Types of Hybridized Orbitals
(See Table 10.3 & Know These)
Hybridization Molecular Geometry
Bond Angle
Examples
1. sp
linear
180o
CO2 N2
2. sp2
trigonal planar
120o
BF3
3. sp3
tetrahedral geometry
109.5o
CH4
4. dsp3
trigonal bipyramidal
120o (base)
90o (apex)
PBr5
5. d2sp3
octahedral
90o
SF6
B) Sigma & Pi Bonding
C2H4 (ethene)
H2C = CH2
has 1  and 1  bond
 bonds - covalent bond where electron pair shared
between two atoms along the bond axis.
 bonds - overlap of parallel p-orbitals
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